After testing the other chips on the card, I discovered that the 7400 chip wasn't fully functional. After that was replaced, my setup worked as it should.
Borrowed some Monitor 80 EPROMs from a friend to compare against mine in an attempt to figure out what was different. It seemed that something was different in the last two EPROMs (at address 020000-021377) as when I used his EPROM card, it worked. However when I compared the ROM contents to mine, they were identical! So there must be something wrong with my ROM card.
After testing the other chips on the card, I discovered that the 7400 chip wasn't fully functional. After that was replaced, my setup worked as it should.
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Yesterday I picked up this Teletype Corporation Model 15 teletypewriter at a local garage sale. Apparently the owner picked it up at a Goodwill auction about 20 years ago for $75.00. He intended on making a steampunk type setup with a raspberry PI or Arduino driver to display twitter or other messages on the machine, but never got around to it.
After some intense negotiation (friendly, but intense), I managed to walk away with it, and a Teletype Modem for $150. He knew it was going to a good home, so I think that helped. I promised to email mail him pictures and links once it was restored and operating. It appears to be missing some components, so it will take some time to restore. There is no SEND/REC/BREAK switch, and both the margin and alert bells are missing. The control relay is missing as well, however it is not clear as to whether or not all this was because it was cannibalized to fix another teletype, or it was a variation in design. This will be fun to play with. I love these old electro-mechanical devices. This one looks a lot simpler, though still fiendishly complicated, than the insides of the ASR33 I have. There is one last mystery to solve with my computer. Fortunately, it has nothing to do with the reproduction boards. It is a problem that appears in both the original and reproduction Mod-80 systems. For some reason, Monitor 80 (the 8080 version of the software) cannot correctly read data from EPROMs in the ZIF socket. When it is asked to do so, it just repeats the same data value over and over for all address locations. Since it cannot read data correctly, it will not be successful writing either as the software verifies the data before moving on to the next address. Thus burning was not tested.
When configured as a Mod 8, the there are no issues. This only happens when the system is configured as a Mod 80. The only difference between the two configurations is the following.
I have three different Mod 80-1 boards, one is a reproduction board, and two are original boards made by two different people at different times. All three exhibit the same behavior, so I doubt the problem has to do with the CPU board. If it were, there would have been some sort of bug fix published, or something in the notes I have, I suspect. Next I checked the Jumper Board. I thought maybe I missed a jumper that affects only this function. However, when the board was double checked with the bus diagram from the Moducomp Mod-8 Supplement no errors were found. As Mr, Sherlock Holmes is fond of saying, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth". This left the two Mod 8-4 boards, or more specifically, the Monitor 80 code I was using, Monitor 8 and Monitor 80 address the ZIF socket slightly differently. In Monitor 8, the EPROM in the ZIF socket appears at addresses 200000 to 200377. Since 200000-200377 could be a real addresses in an 8080 system, Monitor 80, uses addresses P000 to P377 to refer to the EPROM in the ZIF socket. My copy of Monitor 80 was burned using the copy of the code in the Moducomp Mod 80 Supplement. I have checked the ROM contents several times to ensure that they are correct, and found nothing wrong, but this is not an easy task to get right. I suspect there must be a bug in it somewhere. I can't think of any other reason. Can you? I am hoping to get a copy of a original set of ROMs from a friend to compare, and see if that corrects the problem. Stay tuned. At this point, everything tested OK with my completed reproduction version of the Mod8-8 board, except I was unable to burn any EPROMs with it. What appeared to be happening was the 48 V programming voltage was not making it's way past the transistor Q5 (designated T5 in the MF8008 manual). At first I thought I might have been a missing trace, or I had incorrectly installed diode or something like that. Then I recalled that the version of the transistor I used was not in the same packaging as normal, and that maybe the pinouts were different too. Q5 (MJE1103), normally comes in an unusual TO-225 case. The version I bought on the internet (called a MJE1103T) came in a standard TO-220 case. If one views the transistor face-on (with the heatsink behind it), on the TO-225 version, the pins are (from L to R), E,C & B. I could not actually find a datasheet for the MJE1103T, but for most TO-220's the pins are (from L to R) B,C &E. Sure enough, as soon as I reversed this transistor, the proper programming voltage was making its way through, and I was able to burn a 1702A chip successfully. This essentially verified that indeed, the TO-220 version has the pins in reverse order from the TO-225 version.
While reversing this transistor works, it looks kind of ugly, so I will end up ordering the proper MJE1103's. I would recommend avoiding the MJE1103T unless you cannot obtain a MJE1103 for some reason. With that, I can finally declare that the Mod8-8 board fully tested and functional with a few minor modifications. When I do the next run, I will, of course, have those corrected.
Version 0.5 of the Build Instructions for the MIL Mod8/80 Reproduction Kit is now available from this website. It covers some newly discovered issues with the Mod8-8 backplane board as well as added appendices on troubleshooting and how to use a current loop converter to talk to the Mod 8/80 via a PC instead of using a 110 baud teletype. Unfortunately, this board did not appear to work at first. The computer would not communicate with the TTY. After some troubleshooting it turned out that I was missing a trace from between Pin-7 of the TTY Slot and of the CPU Slot. It also turned out that one of the resistors (R59) was incorrectly 100k instead of 100 ohms. Once these were correctly, the board appeared to work.
However, that was not the end of the troubles. I discovered, that although the Mod 8 seemed to work correctly, it could not read 1702 or 1702A EPROMs in the ZIF socket. It turned out that one of the 1N270 diodes (D7), was defective. Once that was replaced. All seemed good. Now the only test that remained was to try and burn an EPROM. For some reason that didn't seem to work. The 48V programming voltage was not making it's way into the socket for some reason. At this point, I am leaving that debugging for another day. I ordered some Cambion 16-pin dip socket through hole component mounting boards to mount the 1N270 diodes used in the diode arrays on the TTY/Reset board. This, turned out to be a much better way of mounting these rare diodes, than bending the diode leads to fit the DIP socket holes. I found these diodes were just to easy to break because the leads were so stiff and the glass so brittle.
This is the last reproduction board that I assembled. I had a working original Mod8-8 board, so I was in no hurry to build this one. I was waiting on some 22 position, 44 contact edge connectors I ordered from Hong Kong. They arrived a few days ago, so I finished the assembly today. In the next few days, I will test it out to see that it works correctly. |
AuthorCharles Baetsen holds a Bachelor and a Master's degree in Engineering Physics from McMaster University in Hamilton, Canada. Archives
February 2024
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