VT-320 interfacing

retrojoe at stateoftheark.ca retrojoe at stateoftheark.ca
Fri Nov 13 15:26:43 CST 2020

Rebuilt the DEC-423 to DB-9F cable with a new db9, and it works a treat.

-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Joe Piche via
Sent: Monday, November 9, 2020 10:34 PM
To: 'Mark J. Blair' <nf6x at nf6x.net>; 'General Discussion: On-Topic and
Off-Topic Posts' <cctalk at classiccmp.org>
Subject: RE: VT-320 interfacing

I built that one, thank you for the link.

Worked for a bit, then quit.

If I move the wires around, I can get it to come and go, so it seems to be
the old db9-F I used is garbage.

I ordered new ones, just waiting for them to arrive.

I'll send an update when I have a new connector.


-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Mark J. Blair via
Sent: Monday, November 9, 2020 7:28 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: VT-320 interfacing

> On Nov 9, 2020, at 5:53 PM, Grant Taylor via cctalk
<cctalk at classiccmp.org> wrote:
>> The VT320 only has the DEC-423 MMJ, no RS-232.
> I /thought/ that Modified Modular Jack was simply a physical thing and
completely independent of Recommended Standard 232.

DECconnect (aka MMJ) interface uses RS-423 electrical signaling, but it is
compatible with RS-232 signaling aside from (I think) some reduced noise
immunity over long lines when not using differential receivers.

The official DEC H8571 DE9 to MMJ adapter is just a plain pinout adapter
which grounds the TX- and RX- signals and doesn't bother with differential
signaling. It drives DSR, CTS and CD on the PC-compatible end with DSR from
the MMJ interface. At the moment, one of our friends here has one for sale
on his Etsy store:


Here's a page with details, including the wiring of that H8571 adapter:


Mark J. Blair, NF6X <nf6x at nf6x.net>

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