On Thu, Jun 18, 2020 at 6:08 PM Peter Coghlan via cctalk <cctalk@classiccmp.org> wrote: > Ethan Dicks wrote: > > 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...
> 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 have an abundance of 1488/1489 chips, but I used to make comms devices. I only have a few MAX232 devices not already baked into projects. > 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? It does need to be the same on both ends, and, yes, a 555 should work, but it's unlikely to be within 1% of a "real" rate. > 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? Yes. Pins 15 and 17 on both ends. 4 loads. You can do asymmetric speeds, but the TX for one end has to match the RX for the other end. I see some discussion of that in this thread, but in my experience, we always had same-room direct connect lines, or dial-up lines over POTS and modem speeds like 1200 bps or 2400 bps because we didn't want to pay for leased or conditioned lines. Every connection I ever set up was symmetric. 99.9% were 56kbps or slower. -ethan
