Ethan Dicks wrote: > On Thu, Jun 18, 2020 at 5:53 AM Peter Coghlan via cctalk > <[email protected]> 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? Regards, Peter. > -ethan
