RS232 voltage level converter box

Tony Duell ard at
Thu Nov 20 01:15:54 CST 2008

> 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.


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