Keyboard inverters/converters for terminals

Fred Cisin cisin at
Fri May 22 12:21:39 CDT 2020

On Fri, 22 May 2020, Tony Duell wrote:
> Of course plugging an RS232 cable (DB25, none of this DE9 nonsense!)
> into a PC printer port (or a PC printer cable into an RS232 port) is a
> good way to let magic smoke out of some TTL chips...

IBM tried to use the [INADEQUATE] protection of opposite gender.  Which 
won't help if there is a sufficient pile of random other cables,
OR, college administrators purchasing "gender-changers" for adding a 
second parallel printer!  (on slot 8 of the 5160)
(For those not familiar, IIRC, slot 8 of the XT/5160 has differences in 
buffering; to avoid accidental misuse, IBM provided the XT with a "free" 
RS232 card to fill up that slot)
Later, when they got 286, 386, 486, and Pentium machines, they dug out 
their gender-changers to see whether the parallel printer would work on 
any male DB25 connectors on those machines.  I wonder if THAT is why IBM 
switched from DB25 to DE9 for serial port.

DE9 was used for 5150 CGA and MDA (not quite compatible)
AND for the early Microsoft "Bus mouse"
and, with opposite gender for "modern" RS232 ports.

DA15 was used for 5150 analog input (joystick) and
some people use it as a specialized video connection.

And, of course, the 5150 used 5 pin DIN for both keyboard AND cassette.
On the back, out of sight.   ("MY left, or YOUR left?")
TRS80 [model 1] had used THREE 5 pin DINs on the back and side by side 
for power, video and cassette.
"Sentience is the ability to learn from misteaks; wisdom is the ability to 
learn from somebody else's previous misteaks."
Around here, IBM never had a cable available for cassette, but the Radio 
Shack #26-1207 worked.

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