Re: Serial card question. (DTE/DCE?)

[dempson@actrix.gen.nz (David Empson)]

Ernest <ernestls@home.NOSPAM.com> wrote:

> "Ernest" <ernestls@home.NOSPAM.com> wrote in message
> news:2NmI7.24106$XJ4.14926857@news1.sttln1.wa.home.com...
> > I found a card called a Perisoft Messenger Asynchronous Serial card with
> two
> > ribbon connecting plugs marked DTE and DCE. What is the DTE/DCE about?
> 
> Nevermind. I figured it out. DTE = Send. DCE = Receive.

Nope.  That might be the effect you get if you use the correct pins on
each connector.

DTE = "Data Terminal Equipment".
DCE = "Data Communication Equipment".

These terms refer to the two standard pinouts used in the RS-232
standard.  The standard is designed so that a device designated as a DTE
can be connected to a device designated as a DCE, using a cable which
connects matching pin numbers on each device (a "straight through"
cable).

The most common DTE devices are printers and terminals.
The most common DCE device is a modem.

A computer is able to act as either a DCE or a DTE, depending on the
type of device you want to connect.  In the case of an Apple II, the
most common serial cards are able to be configured to use either pinout.

For the PC, IBM chose to use the DTE pinout, so a PC can be connected to
a modem (DCE) using a straight-through cable, but a cross-over cable is
required to connect a serial printer (which aren't commonly used on
PCs).

In the case of your card, it sounds like the cable provides two
connectors with complementary pinouts.  You are supposed to use one of
the connectors, whichever one is appropriate for the device you are
connecting.

The question is whether the terms refer to the function assumed by the
Apple II, or to the type of device which is to be connected.


The primary signals involved are the transmit and receive data signals.
For a DB-25 connector, a DTE device uses pin 2 to transmit and pin 3 to
receive.  A DCE device has these signals the other way around.  Pin 7 is
signal ground (for both devices).

There are also a variety of flow control signals, which may or may not
need to be connected depending on the application and the specific
serial hardware involved.