Re: How to plug a WYSE 185ES (serial connections)

From: Richard S. Shuford (
Date: 09/18/03

Date: Wed, 17 Sep 2003 23:23:23 GMT

David le Bourgeois wrote:
| I have a WYSE 185-ES terminal and a classic PC under Linux.
| But I know neither where to connect it nor how. I don't understand
| anything with all that relates to the possibility of "wiring". I
| know serial and parallel port. I see what is a null modem cable
| (DB9, DB25). But I'm not able to do something, because I don't know
| how to plug the device :-( Which sort of cable? I tested with the
| same cable which is useful for my scanner (a null one I suppose?).
| But it seems to me that it is the parallel one, because it is more
| larger than the serial one.
On a typical PC, a 25-pin D-shaped connector (DB-25) may be either
a serial or a parallel port. However, the custom adopted in 1981
by IBM for its first Intel-8088-based PC is that a serial connector
mounted on a chassis is expected to present pins (male), whereas a
parallel connector should be a female type. If you have a mix of
male and female DB-25 connectors on the back of a PC, then,
probably the male ones are serial and female ones are parallel.

On a typical PC, if you have a 9-pin D-shaped connector, it is
usually a serial port, although rarely it might be for a joystick
or something else unusual. Such a connector would properly be
called a "DE-9", yet because shell-size code letter's function is
not widely known, most descriptions call it a "DB-9". Oh, if the
connector name has a suffix letter "DE-9-P" or "DE-9-S", in that
context it means "plug" (male) or "socket" (female), *not* parallel
or serial!
IBM introduced use of the DE-9-P for RS-232-C interfaces in 1985
with the "PC/AT" model (which was based on the 8-MHz Intel 80286

Furthermore, the serial port on a PC should be a DTE-type RS-232-C
interface. If a DTE port is being connected to a DCE-type RS-232-C
interface (as you would find on a modem), a straight-through cable
is used. To connect a DTE directly to another DTE (as connecting
a PC to a terminal), you need a cross-over cable wired as the
infamous "null modem", by means of which the two DTE serial ports
are each fooled into thinking it is connected to a modem.

Some equipment, however, may use female DB-25's for serial ports,
and the gender may not match the DTE or DCE flavor. Your mileage
may vary. No smoking. Fasten seat belts.

A web site with diagrams is here:

Accumulated information on video terminals resides here:

 ...Richard S. Shuford

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