Regarding the TPDD cable and what’s “in” it to handle the RS232 to TTL
level conversions passively, isn’t it possible that the little 3 terminal
devices Marty described are just what he ended up using, i.e.,
diode-resistor dividers in which the diode has a high enough reverse
breakdown voltage not to blow when it’s biased at -5v (most diodes would be
OK with that, I think), and the resistor (22K) is large enough to limit
current (to about 200uA) but also small enough not to act with parasitic
capacitances to “dull” the data signal from the Model 100? At 19.2kbaud,
the parasitics would need to be quite high to impact the signal much, I
suppose, though a long cable would contribute to this risk.
Something like this (ASCII art, anyone?):
TXD: +/- 5v from RS232 M100 side
———
I
I
——-
\. / Diode
V
——-
I
I———-0 TTLrx {~4.4v if TXD=5v, 0v if -5v}
Z
Z Resistor
Z
I
I
/////// Ground
Is this too simplified? Maybe I’m overlooking something. Seems like a
plausible albeit ingeniously simple approach that might describe what
happens to work at least for this specific application. I’m more surprised
that on the return line from TTLtx to RS232 RXD, the 0v signal still seems
to be interpreted as a logical “low” for the M100.
Best wishes,
Anthony
