18 Jun
2020
18 Jun
'20
2:55 p.m.
Ethan Dicks wrote:
On Thu, Jun 18, 2020 at 5:53 AM Peter Coghlan via
cctalk
<cctalk at classiccmp.org> wrote:
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.
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