I came across a very old printer at the thrift store, and got it for a good enough price that it could serve as a distraction so that I wouldn't buy anything else for a few days.
It's a Tandy DMP-203 dot matrix printer. It was pretty dirty, but seemed somewhat well boxed (not original). It had an attached power cord. Initial power-on testing at the store showed it to be quite dead. The ink cartridge was still there.
I was interested in reviving this one. Initial removal of the cartridge was easy enough.
My first concern was seeing why it wasn't powering on. To determine that, I'd have to get inside.
Disassembly
Main cover removal
Warning: panel removal on this device exposes high power circuitry that is potentially lethal. Do not do this if you are not aware of electrical safety. The power board also has high power capacitors that slowly discharge, so do not open the panel until you have had the device unplugged for about a minute.
There is one obvious external screw at the top, rear. This is removed first.
After that's been removed, there are two visible "tabs" that are press-fit / snap-on latches, the tabs attached to the bottom frame. If you press those gently inward, and/or pry the upper case away, each gently lifts up. I used a flathead screwdriver (first picture) to push the tab inward.
As it turns out, most of the case screws are the same.
With the screws removed, I pried the front and sides up. Thankfully, the case did not have additional plastic press-fit clips around the sides or front. But it took additional force to pull up. I'll show why later.
This is the kind of edge connection that's going on inside the front and sides. No clips visible, but there is a little slot in the reinforcement tab.
After doing more prying, the top part popped off with a bit of a snap, and this is why. There is one more press-fit clip underneath the left, middle of the top part. It's about 4.5" away from the left side. In the future, it's probably better to lift up the edge, look inside with a flashlight, stick a screwdriver in the gap, and push that clip a bit to release it.
There may be another way to push on this clip to get it to release. Behind the clip, near the print head area, there is a narrow slot. It may be doable to push something through that, maybe something like a tongue depressor. Oddly, I have something that might work. It's the arm of a pair of macetech programmable glasses. That fits well, and has to reach about 1.5" (38mm) into through the slot in order to press against the tab.
A chunk of one-sided FR-1 PCB board also fits through the slot. You need something long, thin, and strong for this.
Interestingly, the top panel has slots that would allow for a larger control board. There's room for three more buttons, and maybe a few LED indicators. The board, itself, has room for another button, and appears to have unused pads.
This is what it looked like when I had the top flipped over, and the control panel was not yet disconnected.
With the control panel removed, you can see there's room for another button and indicator light. To avoid losing them, I put the screws back into where they originated in the top panel.
Power board removal
Now that the top panel was removed, I could see the power supply board (beige) and a part of the main PCB (green). Both were pretty dirty. The power board has four screws holding it down. Two were blackened somehow, perhaps by corrosion or stray ink.
I removed the four screws holding the power board down.
Then, to remove the power board from the case, it turns out that you have to pry upwards where the on/off switch is. There's a cutout in the plastic of the case that otherwise would appear to be cracked plastic. But, it's there to let the switch clear the case as it's inserted.
Connected to the mains is a fuse. I removed it and tested it, and it's still ok. But if your board isn't powering on, this could be the cause (and could indicate a bigger problem further inside).
You can then gently loosen the main power line around its case clips, and snake it around the rod and pin, and then loosen the piece on the back to provides strain relief.
The power board was connected to the PCB with a 5-wire ribbon and a shrouded connector. (The ribbon also isn't color coded in any way. You kind of have to trust that the bend of the ribbon will help you remember how to put it back together again.) The connector shroud was clipped pretty firmly in place. This is old school stuff, and seems designed to prevent removal. That makes sense, given it's a rattling device.
After some prying with a very small, flathead screwdriver, the shroud popped off like this. The trick is to get the screw under one corner of the shroud and pop it upward. This really only can be done if you have room to get to it, which is why it was necessary to pull the power board away from the bottom frame.
The ribbon wires then are held down with "biting" connectors. You can't just pull them up at this point, and since there are multiple of them, it'd strain things to try to pull them out individually.
Not shown here, but... now you have toothpicks that are stuck. What do you do? Use a small, flathead screwdriver and push each socket tooth. The toothpick will come out, and the socket tooth won't bite into the screwdriver, so it can come out, too.
Power board
Now for a bit of power board inspection. Just as they say on the internet, it looked like there was capacitor leakage. It looked bad and actually smelled a bit like rotten fish.
Here you can see one of the electrolytic capacitors leaked badly...
I took plenty of pictures to try to remember the orientation. But it turns out that the board has good markings on it, showing capacitor polarity.
Off to de-solder. Here are the caps that were removed.
C11 (smallest blueish)
- 4.7 uF
- 6.3V
- 5.16mm diameter
- 11.63mm height
- 5mm lead spacing
C9 (small blackish)
- 330 uF
- 10V ELNA (M) (20%)
- 8.18mm diameter
- 12.36mm height
- 5mm lead spacing
C2 (Nichicon, blue)
- 220 uF
- 200V
- 18.4mm diameter
- 40mm height
- 7.4mm lead spacing (but also can take 10mm!)
C13 (Nichicon, brown/black)
- 4700 uF (4.7F)
- 25V
- 18.3mm diameter
- 34.5mm height
- 7.5mm lead spacing
Armed with that info, I found replacements at both Mouser and Digikey.
One nice surprise on the board is that the board design for C2 could accept either the original through-hole leads at 7.5mm lead spacing, or a snap-in style capacitor with 10mm lead spacing (and 1.5mm-wide, stubby leads). The original PCB had solder flatly laid over the larger holes seen here, since they were disused.
To remove the bad capacitors, I had to cut through a bit of the leaked gunk, and then de-solder. After de-soldering, I did some overall board cleaning. Here are the before and after shots.
At this point, I researched and ordered replacement caps. In the case of C2, I opted to get a similar replacement with 7.5mm lead spacing, only because that's what Digikey was serving up. I'd looked at Mouser earlier, and found that they could have provided snap-in form-factor capacitors only, and at a cheaper price. I went with Digikey in the end, just so I could have all the parts on a single list there.
Frozen print head
Onward with diagnosing the (seemingly) immovable print head.
I tried moving the platen roller, and it rolled as expected, if begrudgingly. However, I couldn't move the printer head side to side (along the X axis) without a lot of force.
I figured the next best thing to do would be to remove the print carriage assembly from the PCB to take a closer look at it. I thought maybe by de-tensioning the belt, and removing the X axis motor, I could rule out motor issues and avoid pressure on the motor pinion gear.
Print carriage assembly and main board removal
The print carriage assembly was held in place by four plastic clips in the lower frame. I thought that a tensioning pin also was getting in the way, but that's actually part of the print carriage assembly and could have been removed later.
After that has come up, the carriage assembly is still entangled by the ribbon cable (be very careful with that), a four-wire ribbon cable, and the two 6-pin motor wire harnesses.
There is a DIP switch panel that I should have removed first. With the print carriage assembly in its original place, you can slide the panel left, up, and out. This untraps the print head ribbon cable and makes subsequent moves strain that ribbon less. This is the view from below.
The four-wire pin assembly is another of those that has a hard-to-remove shroud, and has socket points that "bite" into each wire. I'm not wholly sure what those do. Note that for this one, there is a color coding on one side, so mind that as you reassemble later on.
Here again I used the toothpick trick.
I also marked the header to remember where the gray wire would go.
The print head ribbon cable remained firmly in place. I have not found a way to remove that, so you have to work around it and be very careful to avoid breaking or pinching it.
The six-wire harness to the control panel can also be removed from the PCB using the toothpick trick. The bends in the cable help remember how to put it back in later on. Unclip shroud first.
Toothpicks and removal.
The 6-wire motor harnesses each have their own header, and those can be pried up gently with a small, flathead screwdriver. This is just a picture for color orientation purposes.
The red header came off first (Y axis, or roller axis). I just used a normal screwdriver to twist-pry it up.
Then, the white one (X axis, or print head axis).
It's nice that the headers and sockets are matched by color so I can remember which goes where.
I wonder now if I the ribbon is held in by the same "biting" tooth mechanism as the others. Those work, because they're used tinned, stranded wire that metal tabs can hold onto. Once presented with something more solid, like a screwdriver tip, the teeth can't bite.
Ribbon cable ends would be soft copper. If I were to be able to slide a thin, metal edge into there (perhaps a trimmed aluminum can?) maybe the "teeth" would release from the ribbon.
Here's where things stood at this point. The PCB and carriage assembly are out of the lower frame. In this picture, they're upside-down. The two are held together by the print head ribbon cable. The print head still doesn't move cleanly along the X axis.
In order to remove the PCB, I had to unscrew the screws that were holding it to the lower frame. I also took off the screws that were holding the parallel port (Centronics?) clips, and associated outer flange. I went further than needed, removing the inner mounting brackets. All those parts are seen at the top, right.
X-axis Motor de-tensioning and removal
The next part of the disassembly was to take off the X axis stepper motor. It's seen here, mounted to a metal plate. The plate slides and tension is provided by the spring. In that way, on the opposite side, the motor gear talks to a drive gear that pulls the belt that moves the print head.
The motor is labeled 42SIN-15D6YA, but if you search for that on the internet, you'll likely end up looking at NEMA 17 stepper motors. This is not that. This is a pancake, unipolar, 6-wire stepper. I'm not sure what voltage it runs at and I don't have any other specs on it.
At this point, with the motor completely out, I should have been able to slide the print head freely from side to side, barring there being any "parking" mechanisms. But the print head was still very tough to move. I could move it a bit with quite a bit of force.
I tried adding some sewing machine oil to the X axis rod, and that didn't really help.
So, I started thinking that the X axis could have a sleeve bearing that was frozen, so I started looking at ways to remove the rod entirely from the frame.
This is the blue lever mechanism on the right side of the rod. Looking at it closely, it didn't provide any clues as to how I could remove it.
Looking at it from the inside didn't provide clues, either. I tried unclipping the blue lever away from the frame so it could rotate beyond its normal detente range. I hoped that would get it to a point where it would be released, but it didn't come free, and it felt like I was putting too much strain on it.
Sprocket feed roller removal
So, I started to remove more from the back of the assembly. This is the rearmost assembly that holds the sprockets for tractor-feed paper. I removed it by first taking off the C clip and then sliding off the associated gear. With those off, I could slide the rod to the right, allowing the left end of the rod to come off. Then, I could pull the rod out of the frame, inward from the right end.
Gearing and roller removal
The next task was careful and organized removal of the gears. Some are held in by screws.
With the first set of screws removed, I could slide the beige handle outward from the frame, but could not remove it.
I tried to remove the pin that is used for the roller knob, but that wasn't a good move. (I was able to move it almost halfway out, and then got stuck, so I had to cleverly use a C clamp to gently get it back to where it came from without bending it.)
The next nylon gear was accessible, now that the beige lever was out of the way. This one has a beveled top, and behind it there is a washer and spring, pushing the gear away from the frame. The non-beveled part of this gear (the larger diameter portion) can engage its teeth with the sprocket axle's gear. When the beige gear's "c" arm rotates over the beveled gear, it pushes the gear inward, compressing the spring, and causing the larger diameter gear to disengage from the sprocket axle gear. Here, I remove the beveled gear, washer, and spring.
Going back to the roller axle, right side, there's a C clip holding a gear in place. I removed that.
And now I could slide that gear away from the frame.
That revealed another of those through-the-rod pins, like the one seen under the roller knob. The thing to note here is that the white gear has slots that line up with the pin, so it's important to align that when reassembling.
This is a close-up of one of those through-the-rod pins (photo through loupe). It looks like maybe some kind of spring metal, crimped, and then inserted. If I were to take it out, I might not get it back in again.
In general, I think the through-rod pin mechanism was a cheap way to avoid using set screws. In some ways, given the environment, it may have also been more reliable than set screws.
I still had not freed up the roller from the frame.
It took me a while to figure this part out. It turns out that there are white bushings that have levers on each end of the roller. But, if you look closely, there's a tiny kind of stiff glue holding each lever bushing in place.
The blue stuff is a glue, and its purpose it to keep the lever from rotating around the axle. I used an X-acto knife to cut off the blue and scrape it a bit.
But even with that glue removed, the lever would rotate. It turns out that the lever also has a nib that sets into a hidden hole in the frame. You have to pry the lever tip away from the frame a little in order to release the nib, and then then lever can rotate (assuming you got the glue out).
On the left side of the roller axis, the lever bushing holds the axle in place. Behind it, there's a washer and spring under tension, so those can pop out.
I got the levers away and rotated them to a vertical position. As you do this, you can see that the inner part of the levered bushing has flats on the edges, allowing it to slide upward once in position.
Even with the levers set up properly, the roller wasn't completely free, because of the position of the print head. I forced the print head all the way to the right.
Finally, I could get the roller and lever bushings off. (And, pop! the spring and washer came loose on the left side.)
Here's the right side of the roller assembly. If you look closely, you can see the nib on the lever.
Root cause of frozen print head
Finally with the roller assembly out, I could move the print head along the X axis, and it moved pretty smoothly. But, why? If it was moving cleanly, that meant that the problem was not some kind of frozen bushing, or dirtiness of the X axis rod, though it certainly could use cleaning and oiling.
That's when I finally saw this:
As it turns out, there's little L-shaped clip on the front of the print head carriage, and it was riding on top of the plastic rail at the front of the carriage frame.
That meant that the print head was tipped inward and binding against the roller.
There was no adjustment mechanism for this, so I just pushed that part of the frame inward a bit, and that allowed the L to pop over into its proper position.
Once that was done, I could slide the print head left and right with light finger pressure.
So as it turns out, much of the disassembly done earlier wasn't really necessary, and put the print head ribbon at risk. It did allow me access to a lot more of the machinery, which I could clean.
Reassembly
After that, I could put it all back together again.
A few points on reassembly:
- Be super careful to avoid damaging the print head ribbon. That might be impossible to replace.
- Remember to align the roller's right side gear with the pin.
- It seems like it shouldn't really be necessary to glue the levers in place, but maybe I'll do it.
- The beige lever on the right side has to be rotated upward so that it engages the beveled gear correctly.
- The four-wire ribbon can get trapped in the wrong place under the carriage, preventing proper carriage landing.
- Be sure that the print head ribbon is pulled through the DIP switch panel hole before dropping the carriage in place.
- Before setting the carriage down, there's a lever at its front. That lever has to be held up before the carriage goes down. The lever is part of the X axis homing switch assembly. If the lever isn't moving properly, the print head probably will move left as it tries to home, and will crash into the rigid lever, and might break it. So, make sure it's properly oriented before dropping the carriage back in place, and test manually afterward to make sure you can hear it clicking the tactile switch underneath.
- After the carriage is in place, you should be able to put the DIP switch panel back on. Double check to make sure the print head ribbon is positioned properly before putting the panel on.
- Probably, it's best to keep the motors disconnected (i.e., don't reconnect the red and white connectors to their headers) as much as possible. You don't really want motor movement to provide some kind of flyback current to the board.
With this all reassembled, but with motors still disconnected, there are few other interesting things to note.
- The X axis does move properly it takes more pressure than just a finger push. That makes sense, because the belt is now under tension, and you're pushing against the gearing and the stepper motor. But, it moves without an extraordinary amount of force.
- As the print head moves, you'll see that a gearing mechanism rotates the gears and pin that would drive belt movement. I don't quite understand how that works. As well, it moves them bidirectionally, suggesting it'll keep hitting the same section of the ribbon over and over again, unless something else controls the ribbon drive gear.
Now that all that is done, I'm awaiting the replacement caps so I can get the power board working. I'm hoping that will provide insight into the normal motor voltages. I could try to drive them independently using some kind of microcontroller (Arduino, Polulu motor boards, e.g.) but can't really safely do that without knowing the typical voltage.
Power board repair
I think if I'm reading the traces correctly, pins 1 and 2 on the header are tied; pins 3 and 4 are tied; and pin 5 is separate. I'm getting 1.9v on pin 5, and nothing on pins 3 and 4.
Disconnected, when I tone it out, pins 1, 2, 3, and 4 all produce tone against each other. That's rather unexpected. I guess that suggests (perhaps) some kind of switch that turns on when truly powered.
I checked the traces (using DMM) and they all seem intact.
I'll have to dig deeper into the circuit diagram to try to make sense of what it's doing, and may start detaching components to see if I can figure out if some important transistor, etc., is dead.
Power board voltage (and short circuit) insights
(07-FEB-2026)
A few more insights!
First of all, I took a closer look at the circuit board to see if it would provide clues about voltage on different header pins, and indeed the screen printing did show it!
Front side
It wasn't totally clear where those pins went, so I also looked, and the traces on the back also showed voltages (and you could more clearly see where they went).
It's a little clearer with some Photoshopping and contrast enhancement.
So if both front and back markings are to be trusted, the board is likely set up with the following. The pins for CN1 (connector 1) are number 5 to 1, reading left to right, on top of the board and with the connector at the top. When flipped over, then, the pin solder points are 1 to 5, reading left to right.
Pin 1 = Pin 2 = 24V
Pin 3 = Pin 4 = Ground
Pin 5 = 5V
The other thing I did was check the diodes on the board. I was lazy and didn't disassemble each component. But one of them, ZD2, looked pretty corroded.
As it turns out, all the diodes show some forward voltage drop, except ZD2. That one was showing a short. I de-soldered it and tested it in isolation, and it still shows that it's bad.
The diode appears to be marked 27B, suggesting it's a 27V, +/-5% diode. The diode is 2.7mm thick, and about 4mm long (for the body) with 10mm bent lead spacing. The leads are about 0.8mm thick. The dimensions 2.7mm and 4mm suggest it's a "DO-41" or "DO-41-2" or "DO-41G" case form factor. Some also say "SOD66" form factor. All but one of these listed on Digikey is rated at 1w, and sadly all but one are currently tariffed in the US. Buying a single one from Digikey costs $0.13 for the item, and $6.99 for shipping.
For just over double that price ($15), I picked up a used, apparently working Panasonic KX-P1091i dot matrix printer, and it had two kinds of DB25-to-Centronics types of cables with it. It powers on and didn't require extensive cleaning. So I'm less inclined to spend money on a 27v Zener diode at this point.
I got to thinking: ok, so what does this diode do? It's connected directly between the marked 24v trace and the GND trace in an orientation where the black band is on the GND side. That suggests to me that they're not taking advantage of the reverse bias behavior of a Zener, really. And if it's just there to face a minimal forward voltage drop, maybe its presence drains off some current so it doesn't all go to the 1,2 pins? I don't really understand it.
But, given that I didn't think it'd do anything significant (in my opinion at this point -- need to talk to my EE friends) to the circuit, I thought I'd try powering the board (in isolation, not hooked up to the printer) to see what would happen.
And there you go.
Between GND and 24V, I'm seeing about 24.8 to 25.2V.
Between GND and 5V (pin 5, conveniently), I'm seeing around 7.6V. The 5V line is dangerously high, but the diode that I removed shouldn't really have anything to do with that.
Why is the 5V line so high? It could potentially damage other circuitry, unless the PCB has some kind of power limiting management on it. I don't see any obvious adjustment knobs (potentiometers) on the power board. AI reports back what I suspected. It's not safe, and it could be because of a failed voltage regulator.
At this point, I'm tempted to circumvent the whole power board and put in something that I know is generating 24v and 5v safely. I won't know current draw requirements. The label on the bottom of the printer says 120 VAC, 60 Hz, 0.7A.
It's quite possible that the circuit downstream from the power supply is already damaged, in which case I can try even more severe experiments (drive the motors myself, and then figure out how to fire the dot matrix pins myself).
One thing I can do is trace back from some known chip on the PCB to see where its 5v power line goes, and whether it connects directly to the power board pin 5. One such is the TC51832SPL-12 chip. That appears to be a RAM chip with Vdd at pin 28 (top, rightmost), and GND at the opposite corner at pin 14.
Indeed, pin 14 connects to PCB screw ground, and pin 28 touches power board pin 5 directly. So, yeah, it's possible the power board failed and could have taken some of the PCB chips down. My only safe way to find out, short of diagnosing the 5v line failure on the original board, is to spin up my own power supply.
Alternate power supply
I figured I would try to set up my own 24v power supply and 5v power supply with common ground. I had just received a bench power supply which would fit the bill quite nicely.
I already had an LM2596 DC-DC buck converter.
With some quick wiring, I got the power supply and buck converter talking to each other. A precision pot on the buck converter let me dial its output voltage down to 5v.
Then, with some 22ga patch wire and some quick-connects, I got everything connected to the power ribbon. Again, looking at power wires from left to right, they're numbered 5..1. 5 = 5V. 4,3 = GND. 2,1 = 24V.
I triple-checked all the connections, and also checked again to make sure that the carriage return limit switch was working properly. The moment of truth! I powered it up, the print head homed to the left and hit the limit switch as intended, and the light came on. I could press the "line feed" button and it would move the roller. A press-hold of the "line feed" button caused the roller to scroll the page out.
Not all is perfect with it. If I start with the carriage a few inches from the left margin, it makes an awful grinding noise, and doesn't move the carriage. I think it's quite possible that there's a problem with a bend or a bad tooth in the belt.
Parallel port programming
DMP-203 parallel port timing
I still don't have a DMP-203 manual, so I'm making some close guesses based on a timing diagram seen in a DMP-105 manual.
The back interface for the printer is a Centronics that talks to a DB-25 connector.
There are resources online that translate the pin numbering.
The timing diagram shows that I have to
1. Set up D1..D8 data pins with some value
2. No wait time is required
3. Bring /STROBE low for at least 1.5 usec.
4. Read BUSY. It should be high for at least 500 usec.
5. /ACK will also go low when BUSY goes high.
6. Wait for BUSY to go low and /ACK to go high
7. Again, no wait time is needed before sending the next character D1..D8.
If desired, I can also pay attention to PAPER OUT and FAULT__ signals. Generated pins from the printer are biased high to +5V with a 2.2kOhm resistor. Signal lines sent to the printer are supposed to be biased high with a 1 kOhm resistor as well.
Comparison with Panasonic KP-P1091i interface
The timing for the Panasonic has different wait times.
2. Wait at least 0.5 usec before setting /STROBE low.
3. Keep STROBE low for at last 1.0 usec
4. BUSY signal goes low when /STROBE goes low, whereas for DMP-203, BUSY goes low after /STROBE goes high.
5. BUSY time high ranges, depending on buffer state.
6. When BUSY drops back to low, /ACK drops to low. In general, like with DMP-203, wait for both BUSY low and /ACK high to know printer is ready for the next set of data. /ACK is designated as going low for max 5 usec.
7. With BUSY low, and /ACK rising edge, the next data can be sent.
Significant pins (for my purposes)
All pin numbering shown here is based on a Centronics 36-pin parallel port. There are resources elsewhere with mappings of these to a DB-25 connector. On the DMP-105 documentation, six additional lines are marked NC (Not Connected); three more are marked as 0V without being twisted pair returns; one (pin 13) is the logical opposite of BUSY; and one (pin 17) is CHASSIS GROUND. That adds up to eleven pins that are effectively irrelevant, leaving the other 25 to be usable within a DB-25 connector.
/STROBE (pin 1)
DATA 1..DATA 8 (pins 2..9)
/ACK (pin 10)
BUSY (pin 11)
On both, pins 19..30 are connected to Signal Ground and are set up a twisted pair lines for each corresponding line in pins 1..11.
The chassis ground is on pin 17, and is distinct from the term "0V". This does not appear to have a mapping to the DB25 pins.
On the Panasonic, the +5V line (pin 18) is documented as "...for evaluation only. It should not be used to supply power for external equipment." On the DMP-105, +5 is documented as "80 mA Maximum".
Optional pins
PO (paper out) (pin 12)
/ERROR (pin 32)
/AUTOFEEDXT (pin 14, Panasonic only)
/PRIME (pin 31, Panasonic only)
DB25 pin association
Centronics 1..14 match DB25 1..14
/FAULT = Centronics 32 = DB25 15
NC on DMP-203, PRIME__ on Panasonic, Centronics 31 = DB25 16
NC Centronics 36 = DB25 17
NC on DMP-203, SG (signal ground) on Panasonic, Centronics 33 = DB25 18
Return /STROBE Centronics 19 = DB25 19
Return 2 Centronics 21 = DB25 20
Return 4 Centronics 23 = DB25 21
Return 6 Centronics 25 = DB25 22
Return 8 Centronics 27 = DB25 23
Return 10 Centronics 29 = DB25 24
Return 11 Centronics 30 = DB25 25
Since not all return lines are represented in the DB25, how are they connected to ground? Perhaps there are different styles of DB25-to-Centronics cables. The one I tested is one that came with the Panasonic. That document says, for the SG pin, "The twisted pair return wires (pins 19-30) are connected to signal ground." When the cable is disconnected from the printer, there is no connectivity on those wires. But when the Centronics end of the cable is plugged into the Panasonic, indeed DB25 pins 20..25 all show connectivity to DB25 pin 18.
The same is seen when the Centronics end is plugged into the DMP-203.
Reference: https://en.wikipedia.org/wiki/Parallel_port
Communications circuit
For communication, I can actually use the same board I was using for driving an LED cube. That design was based on having a couple of 74HC595 chips in series so I could shift out 16 bits of data at once. It wasn't really suited to the task of the LED cube because the pin current draw was really too high.
In this case, I'm just driving logic signals, so it's fine.
What I might want, though, is my own set of pull-up resistors or pull-down resistors, depending on the task. The DMP-105 manual indicates that you should use 1kOhm pull-ups for printer inputs, and that printer outputs already are pulled up via a 2.2kOhm resistor to +5v.
Better, though, might be to use just one 8-bit shift register for the data byte. That might save me the complexity of setting up pull-ups or pull-downs on those lines. Really, they don't get read until the /STROBE fires.
So, I'm thinking that STROBE should be pulled high on an output pin.
I can set up a 74HC595N with these pins.
- /OE biased low
- RCLK output low
- /SRCLR biased high
- SRCLK output low
- SER biased low
I "clock out" the serial data on the SER pin, toggling SRCLK with each bit. Toggle RCLK to push all eight bits to output pins, which connect to DATA1..8 on the parallel port.
Delay (0.5 usec or 0 usec)
Toggle STROBE low (1 usec or 1.5 usec) then high.
While BUSY high and ACK low, wait. (Or, handle that in state machine fashion. Do not send data while BUSY high or ACK low.)
So one way to do this might be:
main loop():
check for serial in and buffer
if buffer has a character and not (BUSY_11 or !ACK_10)
set up D1_2..D8_9
delay, if needed
toggle STROBE_1 for appropriate amount of time
continue in main loop
A trickier way to do this might entail setting an interrupt on the ACK rising edge. The problem there is that we don't get a rising edge on the initial state (first character read from serial in) and we do get an ACK on power-up.
An ACK interrupt might say, "If there's a character in the buffer, emit it. On buffer empty, turn off interrupt."
The initial state would say, "If there's a character in the buffer, and the interrupt isn't set, and BUSY is low, turn on the interrupt function and call it right away."
There probably is some kind of timing conflict in here, where data read into the Serial buffer has to occur while the printer is busy. And, we don't want interrupts to cause buffer management (add char, remove char) timing conflicts.
DMP-203 Graphics Mode
I'm hoping the DMP-203 still honors the "graphics mode" mechanism available in the DMP-105. It allows you to specify a "number of dot columns" character style (27, 20 for condensed), go into graphics mode (18), position the head to that column (27, 16, n1, n2; n1 == int(xpos/256); n2 == xpos mod 256) and then top-to-bottom dots (output 0x80 OR (1,2,4,8,16,32,64)).
With the DMP-203, though, I think there must be a different code for printing, because it boasts having 24 dot matrix pins, not just 7.
When done, command 30 exits graphics mode.
ESC/P and ESC/P2?
There is a standard of the era, Epson ESC/P and ESC/P2 format, that correspond to the commands listed above in the DMP-105 manual.
27,20 is "ESC DC4" (device control 4). But, ESC/P used a plain DC4 to mean "cancel double-width printing (one line)".
AI says the Tandy DMP-202 emulates the IBM Proprinter X24 or Epson LQ-850/LQ-570.
So what do those do?
In the Epson LQ manual, it says that 24-pin control is referred to as "triple density" and thus needs 3 bytes of information for each column it prints. The top pin is 0x80 and the bottom pin is 0x01. The output sequence is ESC * m n1 n2 data.
Reference https://files.support.epson.com/pdf/lq850_/lq850_u1.pdf
