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VT101 receive problems

Old Computers

Veteran Member
Aug 30, 2012
I have recently tested my VT101 terminal with a loopback connector. It fails RS-232 loopback test, but it passes the 20mA current loopback test. After testing it a second time with the RS-232 test, the character that represents gibberish received appeared on the screen as I was typing the escape sequence, but it still failed the test. I do know that it transmits the data just fine because the floppy drive in a computer I connected it to activated when I typed the dir command. I am assuming that since the 20mA test passed, the problem is limited to the RS-232 portion. Could there be a loose/bad connection from the DB-25 port to the PCB? The terminal has worked perfectly in the past. The problem has only manifested itself recently.

Old Computers
Update. I just tested the terminal without the 20mA option installed. It passed the RS-232 test. Could the switches on the 20mA board be incorrectly set?
I know that this is an old thread, but I noticed something interesting about my terimnal. The name of the front of the terminal says VT 101, but on the back sticker it says VT100-AA. What do you guys think of that?
I think that the lid got swapped. Imagine a room full of VT1XX terminals, all with interchangable lids. I'm sure it wouldn't matter much to a user what the lid said. In the course of doing some PM cleaning on a bunch of units at the same time, the lids can easily get swapped around.

Okay so it was probably a mixup. Without taking it totally apart is there any way to determine what type of board it has?
Thanks for the help. This will probably also help me to diagnose the problem with the current loop option. The VT-101 service manual link on vt100.net is broken.

I will now try for the VT-100 service manual.
Actually the VT100 service manual is absolutely superb. It goes into such fine detail that it absolutely explains how every circuit in that terminal works. I have done component level repairs on about ten VT100s and I always do so with confidence knowing that the answers are in that manual. It's like having the design engineer sitting at your bench with you. The only other terminal manual I have that is as good is the VT52 service manual. It's manuals like these that are the reason our DEC gear will be running long after most others gear has been given up on.

Sorry for the silly question, but would the VT100-AA differ significantly from the VT100. By that I mean could the VT100-AA lack the external video input jack and have some different positions of the external connectors. The manual, nor printset on bitsavers seem to hint at this.

About the service manuals, I totally agree with you. DEC wrote great documents. My PDP-11's users manual was an invaluable asset for me when I started to take it apart. How many user's manuals give detailed instructions to take a computer apart?
?? Does your "VT100" only have one BNC connector on the back ?? If so, I am wrong about it really being a VT100! A VT100-AA is the plain vanilla standard 120V VT100. But, it should have two BNC connectors, one video in, one video out.

There are two other easy ways to tell as VT100 from a 101. The VT100 main board has a cardedge connector at the lower edge of the board closest to the front of the unit that makes the power supply connections. The cost reduced VT101 has a column of pins that stick out of the power supply and connect to female sockets on the main board toward the rear of the unit. Also, the 100 has a lighter weight switching supply, while the 101 has a heavy transformer in the middle under the CRT.

Here are some pictures of the terminal. Didn't feel like taking it totally apart to check what kind of power supply it is, but I can attest to the fact that it is pretty heavy.

There is no cardedge connector on the board, and the only other connectors are the white connector and the wire that connects to the 20mA current loop option.
Ok, it really is a VT101! Somehow either the base or some stickers got swapped. At least that mystery is solved!

If you are successful in getting a copy of the VT101 service manual, let me know. I'd love to have one for my library.

Thanks for the help. It is very odd that the sticker got mixed up. I bet that the mixup probably happened when it was made.

As for the service manual, there is one on vt100.net, but the link doesn't work.