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

Peter Coghlan cctalk at
Thu Jun 18 16:55:35 CDT 2020

Ethan Dicks wrote:
> 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
> modem.
> 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.

Hi Ethan,

Thanks for your reply.

I can rustle up +/-12V with a bench supply or two but I don't have a
1488 handy.  I should be able to borrow a MAX232 from something though.
I don't have any baud rate generators lying around either.  How about a
555 generating square waves round about 10kHz for something approximating
9600 bps?  Does it have to be spot on a "valid" rate or will anything
"close" do as long as it is the same at both ends?

To be absolutely clear, do I have to drive pins 15 and 17 going to both
interfaces ie four loads on the driver in total?


> -ethan

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