VT-320 interfacing

Mark J. Blair nf6x at nf6x.net
Mon Nov 9 20:27:37 CST 2020

> 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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