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VT320 problems

L

lsi11al

Member
Joined
Apr 20, 2018
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44
Location
Hebron, KY
Now that I have my 11/23+ system running I wanted to try using a real DEC terminal. Connected a VT320 to the computer and all the boot up information appeared on the screen. However, when I went to respond the terminal is apparently not sending to the computer. I have the cable from the terminal plugged into a H8575-A adapter which is then connected to the Dbit cable going to the RS-232 port on the CPU board. I tried connecting pin 2 to 3 on the H8975 but got nothing on the screen. I ran through the loopback tests suggested for the terminal and end up with the VT320 DEC-423 Port Error -- 5. Also tried the printer port and got Error 6. So apparently I only have one way communication - computer to terminal. Anyone have further test suggestions or a possible fix? Or am I just out of luck with this particular terminal?:confused::confused:

Thank you for any ideas.

Al
 
What kind of cable do you have? Often missing CTS/CD/DSR signals (provided by 'real' modems) will prevent sending. Null modem cables will fake these signals. We used to use 3-wire cables between DEC cpu's and serial terminals, which with a 25pin connector required jumpering pins 4->5 and 6->8->20 at both ends of the cable, as well as crossing pins 2-3 between the two ends of the cable.
 
Check the number of data bits and the parity mode. The terminal will probably be a bit more lenient about characters it receives, but the monitor/OS may be very picky. Try 7 data bits, 1 stop bit and vary the parity. I'd try mark parity then odd, even, etc.
 
lsi11al said:
Here are some clarifications on my connections between the VT320 and my 11/23+ system. The terminal only has DEC-423 connectors, no DB25. The attached documents show the pin assignments for the 6-pin DEC-423 connector, the cable used and the adapter to go from DEC-423 to DB25. The adapter then connects to the DB25 connector on the serial port of the computer. View attachment 47622View attachment 47623View attachment 47624
Click to expand...

And when you loop 2 to 3 on the 25pin you get nothing when you type? Is HOLD pressed on the terminal? Is RX and RX baud rate the same?
 
Received a female 6 pin connector today and wired it as complete loop back connector and ran the loopback tests as per the VT320 Pocket Service Guide. Both the communications port and the printer port result in errors. So input to the terminal from the computer works but output to either the computer or the printer port does not. At this point unless someone can point to something I am overlooking I am thinking that there is an internal problem on the control board. Given the directionality symptoms can anyone suggest what chip or chips might be at fault. Also welcome any thoughts on whether this is worthwhile to pursue.

Thank you.

Al
 
lsi11al said:
Received a female 6 pin connector today and wired it as complete loop back connector and ran the loopback tests as per the VT320 Pocket Service Guide. Both the communications port and the printer port result in errors. So input to the terminal from the computer works but output to either the computer or the printer port does not. At this point unless someone can point to something I am overlooking I am thinking that there is an internal problem on the control board. Given the directionality symptoms can anyone suggest what chip or chips might be at fault. Also welcome any thoughts on whether this is worthwhile to pursue.

Thank you.

Al
Click to expand...

Schematics for the VT320 are available on bitsavers in: http://www.bitsavers.org/pdf/dec/terminal/vt320/

Refer to page 11 of the files, schematic sheet P3. Relevant chips are E7 and E10 9636 dual drivers, and E12 2681 DUART.

There are separate driver chips, but the serial channels share a common dual UART device. Could try probing the TTL serial outputs of the UART to see if they are OK, and the driver chips are bad. Or possibly the DUART is bad.
 
billdeg said:
Can you use the terminal with anything else? I know that from a vax 4000 my vt320 needs a null modem adapter or cable.
Click to expand...

He is using an adaptor, and looping 2+3 at the 25-pin plug results in nothing, even though control signals are disabled. I still wonder if the terminal is looking for CTS before sending. Might be worth looping 4+5 as well, and checking that the pin with a voltage on, can't figure out if its 2 or 3 sits at -12v below signal ground (8). If its at zero then I suspect that the 9636 used here. However these seem to be available cheaply.


https://www.ebay.co.uk/itm/292334554008

I know the 75 series RS232 drivers were very prone to popping.
 
The latest loopback I did was directly with the 6-pin plug. I looped DSR to DTR, RX+ to TX+, and RX- to TX-. Those are the only signals available from the terminal. This is what resulted in failure of loopback tests on communication and printer ports. When connecting the terminal to the computer the H8575A adapter does loop pin 4 to 5. I guess the next thing to do is open up the terminal and see what versions of the DUART and Dual Driver chips are on board.
 
I ordered the driver chips, the DUART, and the 40-pin socket. They finally arrived. Had to completely disassemble the monitor to get at the board. Removed the old chips, placed sockets and plugged in new chips. Reassembled the monitor and with great anticipation turned it on. The good news is it now passes the printer port loopback test. The bad news is that it still fails the DEC-423 port loopback test. Not the result I was hoping for.
 
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