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VT101 with constant garbage on screen

fabioNMI

fabioNMI

Member
Joined
Aug 26, 2024
Messages
21
Location
Ingersoll, Ontario, Canada
Hello!

I recently acquired a VT101 and this is my first contact with a terminal like that! Unfortunately it is not fully working and I get garbage on the screen.
I have attached a picture of the screen.
Now this is what I know so far (probing with scope):
1 - voltages are ok and within specs
2 - the CPU (8085) has clock and it is not hold on reset
3 - when I turn it on the ONLINE LED turns on and then after a few seconds the keyboard beeps (1 + 3 short beeps) and ONLINE LED turns off and OFFLINE turns on
4 - there is bidirectional communication on pin 4 of J2A (keyboard) and when I press a key I can see the signal changing
5 - the terminal does not respond to any keypresses (in special SETUP + RESET is not doing anything)
6 - the video shift register (74S299) is hot
7 - removing the ROM (E71) results in no keyboard beep
8 - garbage is CONSTANT, meaning, it is always the same stuff on screen on every power cycle, so it is not just random data from video RAM (which leads me to think the char gen is bad)
Now my questions:
A - when issuing a SETUP + RESET, does the keyboard resets as in step 3 above?
B - where can I find the chargen ROM image?
C - I can't locate jumpers W24 and W25 (for testing the alternate chargen) but I believe I found them just by the resistor near E3 label, is that correct?
Since I don't have spare parts for the video section (DC011, DC012, 8111 memories, 74S299) by battle plan is to start by disabling the main chargen (W6) and enabling alt chargen (W24 and W25) with an erased 2716 installed to check what happens. I am also considering removing E14 SRAM to see if that affects the image output, unfortunately I don't have a way to test these SRAMs.
If any of the experienced troubleshooters here can shed some light I would appreciate a lot! I am really hoping it is not one of the DEC ASICs (DC011 and DC012) as I don't think it is possible to find them anymore...

Thanks!

Fabio
 

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I know almost nothing about these, but a general suggestion could be to check if you get any data on the RS232 (or current loop if it has that interface?) when you press keys.
If so then the circuit that selects if the CPU or video hardware accesses video memory might be faulty.
The repeating garbage on screen seems a bit too repetitive for being random data in video memory that happens to be there when powered on, but rather either the CPU not being able to write the correct blank characters, and/or the readout from video ram.
It's interesting that every second line has 16 back ticks, the letters "ge" and then 22 more back ticks, while the other lines have a pattern of 18 characters repeating, and then 18 identical characters repeating.
I would think that the partial characters are due to "half characters" being displayed, intended to be combined with matching other halves to form double height charaters.
It's also interesting that it shows 40 characters per line, rather than the 80 you'd expect.
Also for the lines not showing "ge" the same weird character is showed on positions where the last four bits of the position number in binary is 0000,0011,0110,1001,1100,1110,1111. Except the last two characters it shows up whenever there are exactly two ones. All this only happens when the upper bits are zero.
 
Put it in LOCAL mode (via a top-row key on the keyboard) and then every key you type should echo on the screen (IIRC, it's been about 20 years since I typed on a VT-10x terminal). That would be a good test to do first.
 
Thanks for the suggestions!
@MiaM great insights! I am considering making an adapter to replace the two 2111/8111 with a single 6116 SMT, but if the line buffer is not bad, then I am afraid the fault would be on the DC11 and that would mean a dead end... :( There is still a lot on investigation to be done though!
@pbirkel@gmail.com thanks, I have downloaded the ROM image!
@GreyHairedDECfan regarding LOCAL mode: as I mentioned before, the terminal is not responding to keyboard commands such as SETUP + RESET so I don't think LOCAL is gonna work, but I will have a look! Thanks!
Yesterday late night I also noticed that while the bi-directional stream is present on pin 4 of J2A, there is no RX data on pin 20 of the 6402 UART, so there is something wrong with the LM393 or around it... This not related to the garbage on the screen but probably explains why the terminal is unresponsive to keystrokes!
Thanks for the help so far and keep it coming! I want to bring this terminal back to life!
 
fabioNMI said:
Thanks for the suggestions!
@MiaM great insights! I am considering making an adapter to replace the two 2111/8111 with a single 6116 SMT, but if the line buffer is not bad, then I am afraid the fault would be on the DC11 and that would mean a dead end... :( There is still a lot on investigation to be done though!
Click to expand...
If you would make an adapter for a 6116 chip. but for a through hole mount 6116, then a somewhat weird test you could do would be to instead use an (E)EPROM filled with some test pattern, and see if it that displays correctly on screen. Not sure if that would be that meaningful, but still.

Btw, any signal on the "host" connector when pressing random keys (with hardware handshake wires connected in a way that makes it send even if hardware handshake is on)?


Also: Since it's a regular 8085 processor, you could in theory create some sort of diagnostic ROM that can use the host port to show diag info even if all ram and the display isn't working.
(I think the code is Z80 specific, but an inspiration might be the diagnostic ROMs that people developed helping Adrians Digital Basement (Youtube) to repair various TRS 80 model 1, 3 and 4 computers).
 
MiaM said:
If you would make an adapter for a 6116 chip. but for a through hole mount 6116, then a somewhat weird test you could do would be to instead use an (E)EPROM filled with some test pattern, and see if it that displays correctly on screen. Not sure if that would be that meaningful, but still.

Btw, any signal on the "host" connector when pressing random keys (with hardware handshake wires connected in a way that makes it send even if hardware handshake is on)?


Also: Since it's a regular 8085 processor, you could in theory create some sort of diagnostic ROM that can use the host port to show diag info even if all ram and the display isn't working.
(I think the code is Z80 specific, but an inspiration might be the diagnostic ROMs that people developed helping Adrians Digital Basement (Youtube) to repair various TRS 80 model 1, 3 and 4 computers).
Click to expand...
I did not have time to go back to the VT101 yet but I will take another look later today after work.
Yes, writing a custom firmware for it is doable but it will be a lot of work, maybe one thing that I can try before that is using a Fluke 9010a with 8085 pod to inspect the buses and memories, but I am still working on bringing the Fluke back to life so... lol Lots of work...
Regarding the "host" connector, I don't think it is going to work since there is no RX data on the 6402 RX pin, I will have to dig and find out why, maybe the LM393 is dead (ordered new ones) or maybe there is something else. We will see!
 
It seems like MAME can emulate a VT100 terminal, so creating some sort of test rom (to start with maybe just send 00-FF in a loop on the serial port, to be able to tell if the processor runs at all) might not be that hard, perhaps?

A problem with emulators is that they might ignore initialization requirements. A tangent that shows this is that afaik the MAME emulation of the Ericsson PC can run regular PC compatible BIOS ROMs while IRL you need the special Ericsson BIOS to initialize the not fully PC standard keyboard controller. (It's compatible enough that afaik all PC software works fine, but the hardware interface differs in addition to it requiring special initialization). In theory you might end up creating a VT100 test/diagnose ROM that only works in emulation. Not super likely if you check the data sheets / schematics for things, but still.
 
MiaM said:
It seems like MAME can emulate a VT100 terminal, so creating some sort of test rom (to start with maybe just send 00-FF in a loop on the serial port, to be able to tell if the processor runs at all) might not be that hard, perhaps?

A problem with emulators is that they might ignore initialization requirements. A tangent that shows this is that afaik the MAME emulation of the Ericsson PC can run regular PC compatible BIOS ROMs while IRL you need the special Ericsson BIOS to initialize the not fully PC standard keyboard controller. (It's compatible enough that afaik all PC software works fine, but the hardware interface differs in addition to it requiring special initialization). In theory you might end up creating a VT100 test/diagnose ROM that only works in emulation. Not super likely if you check the data sheets / schematics for things, but still.
Click to expand...
I have never written code on MAME but I agree with you it could be a way to go. I also have a TRS-80 model 102 here that uses the same 8085 (ok not the same one since the model 102 uses a 80C85), so I am thinking that maybe it could also be a coding platform for simpler stuff. Well, since the idea is just do the 8085 at first, it would be easier to simply use an 8085 simulator such as https://www.sim8085.com/ lol
 
Back to this topic, I have now tried the following (all ICs except for E71 ROM are soldered so I am installing sockets as I go):
1 - +12 Voltage was reading +15.4, checked capacitors on the PSU, checked the error amplifier and voltage dividers, in the end, replaced the main transistor Q1 (2N3791) by a MJ2955 and the PSU now outputs 12V as it should
2 - Replaced the 8085 CPU
3 - Dumped the E71 ROM so check if it had any signs of issues (such as one of the data lines stuck). All seems ok but my ROM version is different than everything else I've found on the Internet
4 - Tried other ROM images from the Internet (on a 2732). Different pattern but no keyboard init, no beeps and no LED changes
5 - Removed, tested and replaced E59 (MK4118) by a 6116
6 - Removed and tested E20 (7417), I don't have spares of this one
7 - Removed, tested and replaced E48 (74LS04)
8 - Removed, tested and replaced E72 (74LS367) - buffer for control signals including IO/M
9 - Removed, tested and replaced E33 (74LS367) - buffer for signals including MEM RD L
10 - Removed, tested and replaced E60 (74LS139) - memory address decoder
11 - Tried the ROMs on the Internet on MAME's VT101
I started replacing some of the ICs because I was following what looked a contention happening on MEM EN L signal where I was seeing on the scope a good ~4V tall pulse and sometimes a smaller ~2V pulse and following the path of IO/M and MEM RD L led me to those ICs. After replacing them, that smaller pulse seems to be gone, but the display issue is still the same.
Conclusions so far:
A - The firmware seems to be running fine from the original ROM, the beeps I hear on start up are the same I hear on MAME and the LED sequence seems to be the same
B - Because the buzzer and LEDs are working on the keyboard I am assuming most of the keyboard circuitry, including the 6402 are ok
C - Running the terminal without the E71 ROM causes the pattern to change completely
D - Running the terminal without the E59 RAM also causes the pattern to change
E - E5 (74S299), video serial shifter runs hot but I think that is common and not the issue here
F - tried piggybacking a 6116 on each of the SRAMs, the pattern changed for E59 and E57
I still don't understand why the ROMs that I found on the Internet work on MAME but not on my VT101. Moreover, reading the ROM on my VT101 shows strings such as "SETUP", "WAIT" and the standard baud rates, but I don't see any of that on the ROMs I found on the Internet. Still, those ROMs work on MAME... I have also attached the ROM dump from my VT101.
Next up, I am planning on removing and testing the other two SRAMs (MK4118).
Any other ideas?
 

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I have a hot video shift register too on my VT100. I wasn't sure if it is a problem or not? Is it considered normal?
 
rjarratt said:
I have a hot video shift register too on my VT100. I wasn't sure if it is a problem or not? Is it considered normal?
Click to expand...
Yes I think so. 74S series is faster on the expense of power consumption. While a 74LS299 has a typical power consumption of about 33mA a 74S299 draws about 140mA! I have seen othe 74S ICs running hot on other devices so I believe this is not an issue.
 
Update: I just programmed an MCM68764 EPROM on an old EP-1 programmer here and yes the ROM image from the Internet (23-028e4-00.e71) gives me the same result, as the original ROM! That means it works but so does my original ROM and the problem is elsewhere...
Next up I will remove the other two MK4118s!
 
Some more news:
All the MK4118 memories are good, replaced them with 6116s and the problem is the same.
So now knowing that CPU is ok, ROM is ok and CPU RAM is ok, I've decided to inspect the video generation circuitry more closely, in special the data path from the 4118s (which store the video data) to the actual video generation. As part of that data path there is a 74LS273 (E27) which is the screen latch. Probing all the pins of that IC I found that pin 9 (an output) was stuck low. Replaced the 273 and finally... The pattern changed! Well, that is progress!
I also probed E28 (72LS244) and found that its pin 7 was exhibiting weird signals, some pulses with the right amplitude (>4V) and some pulses with low amplitude (~2V), further analysis is showing that the low amplitude data is coming from the video line buffer (a 2111A SRAM).
Unfortunately I don't have spares and it seems they are not easy to find so I am considering designing a small PCB with a 6116 to replace both 2111As. Any other ideas?
 
Steve Toner said:
I think you gave up too quickly on finding 2111A replacements. Arcade parts suppliers have them (one example given in link). Also, the 6561 (also here) or IM65X61 parts appear to be pin-compatible (but CMOS instead of NMOS), so may work as a replacement.
Click to expand...
Thanks, yes I saw Arcade parts but at $11.50 per part plus shipping I think my option is simpler, cheaper and hopefully more reliable! We will see!
Edit: I also took a look at the 6561 option you linked, it looks interesting! But again, about $10 USD + $30 USD shipping to Canada! For now I am proceeding with my replacement...
 
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