DEC VT320 Low TX Voltage

[struja]

I have a DEC VT320 and it's able to receive but not send data to another RS232 device. I checked the signal on the TX+ pin and it's showing as 1V ±. Is this normal?

 

[Steve Toner]

No, it's not. Should be at worst around ±5V.

I'd start by checking the ±12V levels on pins 5 and 8 of E7:

 

[struja]

ping 8/5 voltrages looks good 12+-
pin 3 - 5v (input) 2A
pin 6 - 1v (output) 2Y
Bad UA9636AC?

Pin 3


Pin 6

 

[gwiley]

Could be a bad E7, or something is loading it down. Can you easily lift one lead of D26 to see if the amplitude of TX data increases on E7 pin 6?

 

[daver2]

I would measure pins 2 and 7 as well.

This is the other device in the same package driving the DTR handshake signal.

Agree with making sure D26 hasn't gone bad.

Dave

 

[struja]

It's working now! D26 was the issue. Thanks for the help!

 

[daver2]

Excellent.

So something (probably external) at some point must have killed it. Or you were just unlucky...

Dave

 

[Bitly]

FYI replacement diodes are available from Digikey for 0.49 each (plus like $5 in shipping, grrr...). I'd definitely replace the bad one (maybe both) to protect the 9636 from future ESD hits.

CW

 

[struja]

Already on order https://www.digikey.com/en/products/detail/littelfuse-inc/P6KE22CA/688080