Hardware Modifications – TRS-80 Model III

Important Note

All modifications are COMPLETELY at your risk. I have not the slightest idea if any of them work. If you decide do to any of this stuff, and it damages your hardware, that’s on you. Proceed at your own risk.

Hardware Modifications and Patches – Menu

Connecting an ATARI 400 to a Model III – Dan Hamilton

I bought an ATARI 400 and quickly got sick of that damn monopanel keyboard. Even with communications gear, it was a hit or miss proposition getting copies of programs off CompuServe, because with the small memory I must use unsophisticated terminal software. Little errors in file transfer were causing big problems.

To overcome that I tried to find a way to use my TRS-80 Model III for the download portion, because it supports the CompuServe A protocol, has gobs of memory, and runs circles around the 400 except in the areas of sound and graphics, where the Atari makes the TRS look mighty poor. I assume similar transfer between an ATARI 800 and Model III is possible too, but I doubt there is any compelling need.

So, I made up a cable to go from port #1 on the 850 interface to a black box, which looks fancy, but merely provides a place to hang two DB 25 connectors and some wire.

Here is how to wire two female DB 25’s together, and how to make the cable from the 850 to the black box.

  1. Cable
    • Hook pin 1 on a male nine-pin plug to pin 20 on the male 25-pin plug
    • Hook pin 2 on a male nine-pin plug to pin 8 on the male 25-pin plug
    • Hook pin 3 on a male nine-pin plug to pin 2 on the male 25-pin plug
    • Hook pin 4 on a male nine-pin plug to pin 3 on the male 25-pin plug
    • Hook pin 5 on a male nine-pin plug to pin 7 on the male 25-pin plug
    • Hook pin 6 on a male nine-pin plug to pin 6 on the male 25-pin plug
    • Hook pin 7 on a male nine-pin plug to pin 4 on the male 25-pin plug
    • Hook pin 8 on a male nine-pin plug to pin 5 on the male 25-pin plug
    • Don’t hook pin 9 on a male nine-pin plug to anything
    • If you already have an Atari modem cable it is likely to be wired this way already.
  2. Black Box
    • Wire two female DB 25’s together in the following manner. (note: Model I users can skip this part; a TEL/COM switch on your RS-232 swaps pins for you). The object is to end up with pins 2 and 3 swapped,
    • Wire from one connector’s pin 2 to the other’s pin 3
    • Wire from one connector’s pin 3 to the other’s pin 2
    • Wire from one connector’s pin 4 to the other’s pin 4
    • Wire from one connector’s pin 5 to the other’s pin 5
    • Wire from one connector’s pin 6 to the other’s pin 6
    • Wire from one connector’s pin 7 to the other’s pin 7
    • Wire from one connector’s pin 8 to the other’s pin 8
    • Wire from one connector’s pin 20 to the other’s pin 20
  3. Connect
    • Hook the cable from the 850 to the black box, and hook the cable from the Model III RS-232 port to the other end of the box.
  4. TRS-80 Software
    • Now just fire up a smart terminal program on your TRS. I use STERM, available on many BBS’s, because it allows me to select speed of transmission and gives me a huge buffer.
  5. Atari Software
    • Get MINITERM (from XA2) or, if you have more memory, use one of the fancier programs like JTERM or AMODEM, and you are ready to go.
  6. Download
    • Go online and use the VIDTEX Model III exec to download error free copies of SIG programs.
  7. Transfer
    • Fire up the terminal program on the Atari, turn on the capture buffer, (no pouncing on the START key because YOU choose when to start sending) and watch the goodies flow onto the screen in living color from your poor old monochrome TRS.
  8. Save Buffer
    • Then, simply save buffer contents the way your program specifies. You can even use the TRS keyboard to enter programs into the ATARI (as long as ATASCII characters are not needed or can be added later) by using the echo mode on programs like STERM.

Upgrading a Model III to a Model 4

The process itself was not difficult, but I did encounter a couple of ‘pitfalls’ that resulted in not getting a working computer back until Tuesday evening (yes, I was getting worried!). To start at the beginning, when I visited the tent sale I noticed a large display of cassette based Model 4’s on sale for $50.00. After picking out one that seemed to have all of its parts intact and lugging it into the store and plugging it in to see that it brought up ROM BASIC ok as evidenced by the familiar ‘Cassette?’, ‘Memory?’ and ‘Ready’ prompts, I paid my money and took my prize home.

My first thought was to simply install the Model 4 motherboard and keyboard in my Model III. After some consultation and reflection, however, I decided it would make more sense to move the drives, drive controller, and RS232 board from the 3 to the 4. This offered the benefit of taking advantage of the newer electronics in the 4 and it would LOOK like a 4. With this plan in mind, I disassembled both machines and set about the task of removing and installing parts. The first difficulty I encountered was the fact that the ribbon cables that connect the floppy drive controller board and the RS232 board to the motherboard were different for the two machines. On the III, the connectors at each end of the cable are exposed on the same side, while the 4 required that the connectors be exposed on opposite sides. This is a function of a difference in the plugs on the motherboard.

The plugs on the III have teeth on both sides of the plug, while the 4 plugs had teeth on only one side. The original cables are also a bit flimsy and don’t lend themselves to successful modification. This meant putting everything away for the night with plans to call National Parts the next morning. The young lady I spoke with at National Parts was quite helpful and looked up the part number for me. She explained that they could ship the new cables that day and I would have them in two or three days. I explained to her that she was speaking with a desperate man and asked her to try to visualize my den floor with parts from two computers scattered about. She very kindly arranged for me to pick up the cables that afternoon at the Northside Drive outlet store (next door to National Parts). After picking up the cables (about $7.00), I headed home confident that my ‘new’ Model 4 would be soon be up and running!

Here’s where the REAL problem began! After reassembling everything and double checking all the connections, I plugged in the 4 and hit the power switch. No smoke, but no drives either! The Model 4 simply refused to recognize that the drives were there! I disassembled EVERYTHING and put it all back together slowly,rechecking every connection as I went – with the same result! The machine would come up in cassette basic just fine, but the drives wouldn’t spin. More phone calls and consultations. Finally, it dawned on me that I had forgotten one of the basic rule of electronics (though I had been warned): Make sure nothing is grounded where it shouldn’t be!

The culprit in this case was the floppy drive controller board! It didn’t have the insulators on the back. The mounts on the Model 4 chassis are slightly different from their counterparts on the Model III chassis – enough different to cause a ground (short). I fabricated suitable insulators, reinstalled everything and the 4 booted just fine with drive 0 spinning and the ‘Diskette’ message appearing on the screen.

I did have to make one more trip, this time to the auto parts store to get a supply of wire ties before putting everything back together (those wire bundles looked like they could be a real problem if not securely fastened).

Would I do it again? You bet, but I’d do it a little differently of course as a result of what I learned in the process. The end result is very much worth the investment of $57.00 and the time required to swap parts. I’ve only been using the 4 for a few days now, but I LIKE it! From a hardware standpoint, the Model 4 keyboard is greatly improved over the 3 and of course the 80 by 24 display is much more convenient. Model 4 software (public domain) seems to be much more plentiful as well and you can still run Model III software in the Model 4’s ‘Model III’ mode.

If you decide to tackle such a project, I’ve included some tips below that you might find helpful.

  1. Get the required parts first:
    • 20 conductor ribbon cables to connect the floppy drive controller and the RS232 board to the Model 4 mother board. Two are required and they are identical (Radio Shack National Parts).
    • Wire ties. (Auto supply store)
    • Insulating washers. (You can fabricate these yourself by cutting them out of a sheet of thick plastic or other suitable insulating material)
  2. Find something else for your six year old to do instead of ‘helping’!
  3. Stake out an area you can control. (I chose the floor of my den, but leaning over on your hands and knees does get painful after awhile.)
  4. Carefully disassemble the machine by placing it on its side and removing the screws from the bottom (10) and the middle of the back (1). Three different size screws are used on the bottom so keep track of which screw came out of which hole. After the screws are removed, right the machine and GENTLY lift off the top. The back of the CRT is vulnerable in this step. You must lift the cover straight up or angled slightly towards the front so that the back of the tube will clear the wiring and the aluminum shield housing the boards. On your Model 3, you can look through the drive 1 hole to make sure it clears. If you feel resistance, don’t force it. Once the cover is up and the CRT is clear, lay the cover over on its side to the left of the base. There are two connectors (one ground, one card edge connector) connecting the video to the base. Disconnect these and place the top in a safe place out of the way.
  5. Remove the screws holding the aluminum panel in place and remove the panel exposing the mother board.
  6. Disconnect all the plugs around the edges of the mother board.
  7. Remove the screws holding the mother board in place and gently remove the mother board, setting it aside in a safe place.
  8. After the above steps are performed on both machines, disconnect and remove the drives from the 3.
  9. Unplug the power supply on the side of the drive tower and remove the drive tower. Reinstall the drive tower in the 4 case and reconnect the power supply (make note of the orientation of the plug in the 3 and plug it in the same way in the 4).
  10. My 4 had some type of network board installed behind the mother board. If your’s is the same, remove this board and all associated wiring (here’s where you’ll have to cut the old wire ties).
  11. Remove the floppy drive controller board and RS232 board from the III and reinstall in the 4 making sure to insulate between the back of the board and the aluminum frame. The RS232 board plugs into the main power supply and the FDC board plugs into the power supply on the drive tower. Plug your new ribbon cables into these boards making sure that the exposed conductors will be in the correct position to contact the model 4 board’s contacts when the cable is looped over the top of the board (opposite from the side they were on in my 3).
  12. Reinstall and reconnect the drives.
  13. Reinstall the model 4 mother board (again making sure of proper insulation behind the board) and reconnect all plugs.
  14. Secure all wiring with wire ties.
  15. Using a pair of wire cutters, cut the plastic around the edges of the plastic blocks covering the drive holes in the top of the Model 4 case. Gently ‘break’ out these plastic covers, remembering that you will want to use them to cover the holes in the 3.
  16. Reconnect the two leads to the Model 4 video. This would be a good point to plug in the 4, cross your fingers, and apply power. If all is well, drive 0 will light up and spin and the monitor will display the ‘Diskette’ message. On the 4, the drive will keep spinning until you insert a disk. Insert a system disk and make sure it boots normally. If it doesn’t perform as above, its time to unplug and go back and check all your connections.
  17. If you got through step 14 ok, replace the aluminum cover over the boards and replace the top (again making sure you don’t damage the back of the tube), replace the screws, and begin enjoying your new Model 4. Don’t get so caught up in the 4 that you forget to put the 3 back together, remember, it’s still a good machine and can run cassette based software (a present to your six year old for staying out of the way?).

BUT ….

According to page 5 in the hardware section of the manual, a motherboard configured for 16K operation (jumper positions and capacitors) uses three voltages (+12, +5, -5) for 16K RAM Chips while 64K rams require only +5. When power is applied to the replacement motherboard with the jumpers and capacitors positioned for 16K operation, the 64K rams will get ‘zapped’ when hit with +12 volts. Modify the board to 64K operation before using non-16K chips or you WILL lose all your RAM chips!

Keyboard Repair – PDP11 Hacker

  1. Unplug everything (esp power cable), and let the CRT discharge.
  2. Remove top cover of the machine, the keyboard bezel (screws round the edge), unplug the keyboard, remove it
  3. Remove all the keycaps (they just pull off).
  4. Remove the screws from the track side of the PCB
  5. Now for the tricky bit. Unsolder all the keyswitches from the PCB. I found the easiest way to do this was to remove the solder with a solder sucker from each switch in turn, and then to pop the switch out of the metal frame. Start with the ones round the edge. When you have got them all free, the PCB separates as well. You can remove a switch from the middle of the keyboard if you are careful. Pull off enough keycaps to get to it easily, unsolder the pins, and then you can release the clips holding the switch to the metal frame with a screwdriver while pulling out the keyswitch with pliers.
  6. Each switch can be uncliped into 2 parts. You can then see the contacts
  7. When reassembling, I found it best to fit all the switches to the metal frame, put the PCB over them, screw it down, and then solder them

If you are stuck for a keyswitch, move the bad ones onto the numeric pad, and use the ones from there (which tend to have little wear) onto the main part of the keyboard. That gets you 12 good switches.

Do NOT use a spray cleaner, or attempt to pop off the keys like on a Model I. These solutions do not work for a Model III.