On Thu, Jun 18, 2020 at 5:53 AM Peter Coghlan via cctalk <cctalk@classiccmp.org> 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. -ethan