19 Nov
2008
19 Nov
'08
11:15 p.m.
According to doctor Marty, the Tandy portable disk
drive is serial TTL
(He says that there are diodes and resistors inside the DB25 hood of the
"special" cable)
Quite possibly. It was intended to be used with the M100 series only,
IIRC, and those machines had RS232 receivers (1489s) that would accept TTL
levels. I will admit that I've driven such RS232 ports directly from TTL
outputs for quick kludges on my bench, but never for anything I expect to
be used by others (I have no way of knowing what sort of port it is going
to be linked to).
I've seen several RS232 peripherals, including the Dowty Quattro modem I
used to use (back when 2400 bps [1] was fast) which take RS232 signals,
feed then through resistors straight into 74HCT inputs, relying on the
protection diodes of said inputs to clamp the signal. It clearly works,
but I don't like it, and would never do it myself.
-tony