Keyboard inverters/converters for terminals

Tony Duell ard.p850ug1 at
Fri May 22 12:28:12 CDT 2020

On Fri, May 22, 2020 at 6:21 PM Fred Cisin via cctalk
<cctalk at> wrote:
> 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,

A lot of manufacturers, including major ones used the wrong gender of
connector on their RS232 ports... A socket wired as a DTE is common.

The HP150 is a classic... HP did just that (2 serial ports, wired as
DTEs, but sockets on the main unit). There was an option board which
included a parallel port. It used a DB25 _plug_. But it gets better,
The PCB was clearly designed for a DB25 socket, which would have had a
pinout compatible with the IBM parallel port. It appears that at the
last moment HP changed to a plug (I guess so you couldn't plug the
official HP cables into the wrong connectors). The result is that the
pins are mirrored compared to IBM, strobe is on pin 13, Data 0 on pin
12, etc...


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