Synchronous serial Re: E-Mail Formats RE: Future of cctalk/cctech

Ethan Dicks ethan.dicks at
Thu Jun 18 13:52:45 CDT 2020

On Thu, Jun 18, 2020 at 5:53 AM Peter Coghlan via cctalk
<cctalk at> wrote:
> To get somewhere near back on topic, I am trying to set up a synchronous
> serial link between two MicroVAX 3100 machines with DSH32 (or DST32 maybe)
> interfaces.  One of the options I have is a BC19D cable and a BC19V cable
> which seem to be identical or nearly identical.  Each plugs into a DSH32
> at one end and has a V.24 DB25 connector at the other end.  I don't seem
> to have anything available in the way of a pair of suitably similar modems
> or a modem eliminator to put between the two V.24 connectors.  Can anyone
> suggest some kind of a quick hardware hack that I could use to fill the
> gap?  Is a pair of DB25 sockets with crossed over wiring betweeen them
> sufficient or do I need something that generates clock signals too?

If both ends don't care about delays in the handshake lines that would
be natural with a modem or high-end modem eliminator, you can just
match up the signals between the two devices as you would for a null

As for the clocking, yes, a modem or modem eliminator provides the
baud rate clocking on pins 15 and 17.  You could use any one of a
number of baud rate generators, from the COM 8116 (one that we used at
work in the early 80s for a simple modem eliminator) to a modern
microcontroller thumping out pulses at the right frequency.  You'll
need to drive both sides of the connection at RS-232 levels, so a
level shifter (1488 if you have +/-12V handy, or MAX232 if you do
not).  AFAIK, you can drive both ends from one line driver, but the
safer course would be to drive each clock pin independently.


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