18 Jun
2020
18 Jun
'20
3:33 p.m.
On Thu, Jun 18, 2020 at 6:08 PM Peter Coghlan via cctalk
<cctalk at 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