18 Jun
2020
18 Jun
'20
5:21 p.m.
On 6/18/20 2:55 PM, Peter Coghlan via cctalk wrote:
Ethan Dicks wrote:
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?
Sync, in my recollection, is a bit fussier than async with respect to
the clock signal. Unlike async, which "samples" the bit cell
more-or-less in the center (hexce the 16x or 64x clock rate),
synchronous is edge-sampled, so that the clock signal must match the
data signal with respect to phase.
The clock signal for receive data is recovered by the receiving modem
and so is always in-phase.
Such stuff as protocol details can differ greatly (e.g. Bisync vs.
SDLC), but remember that there are no start or stop bits like as in
async. Synchronization is done by pattern-matching with "idle" bits
being sent to fill gaps or to form preamble to sync. In some respects,
not terribly different from hard disk recording.
It's been years since I fooled with the stuff, so I may have quite a bit
wrong.
--Chuck