Category : Tutorials + Patches
Archive   : RS232CBL.ZIP
Filename : RS232GEN.TXT

RS232 GENERAL CABLES
January 7, 1985

This write-up presents generally useful RS232 cable testing
information and configurations. Point 4 explains a major danger
that your equipment is exposed to when from customizing cables.
This exposure is critical and over-rides all other points: if
the situation seems to suggest that you act contrary to the
explanation in Point 4, you should have someone else review
the situation with you.


**********Copywrite*********
This material is copyrighted by Global Data Processing
of Dallas, Texas. Permission for non-commercial use, such
as computer bulletin board uploading/downloading is granted,
provided that this notice is preserved intact. Any commercial
use or inclusion must have written permission. Contact Global
Data Processing for written permission: (214) 243-8744.
**********Copywrite**********

**********Cautions:**********
No warranty or suitability of use or responsibility is assumed
nor implied by the author(s) or publishers or repository agents
of this write-up.
**********Cautions:**********
Mis-cabling or incorrect connections can cause permanent damage
to equipment, requiring expensive repairs or replacement. This
damage typically is not covered by equipment warranties. I now
own a dead printer, ruined by a mis-connection.
**********Cautions:**********
While there is specifically-designed test equipment that permits
signal lights and voltage level displays, ordinary voltmeters or
signal lights can cause permanent damage to computer equipment.
For example, current drain can be too heavy by some signal
lights that use your computer equipment signals to illuminate the
lights.

When I am figuring out a new cable problem, I borrow a
professional break-out box which does not drive signal lights
from the computer device voltages/current (it detects signals
and has batteries to light the signal lights).
**********Cautions***********


1. Determining equipment type
A full duplex device will generally assert either the TX
(transmit) or RX (receive) line. An "assert" is a "mark"
(a voltage of less than or equal to -3 volts) or a "space"
(a voltage of greater than or equal to +3 volts).

If TX is asserted, then the device is Data Terminal Equipment
(DTE) and generally configured to receive from hosts and
modems. If RX is asserted, then the device is Data Set
Communications Equipment (DCE) and generally configured to act
as a host or modem.

DTE equipment will also typically assert Ready To Send (RTS,
pin 4) and Data Terminal Ready (DTR, pin 20). DCE equipment
will also typically assert Clear To Send (CTS, pin 5), Data
Carrier Detected (DCD, pin 8), and Data Set Ready (DSR,
pin 6).

2. Configuration note:
If a device asserts a given signal, you should generally
expect that it requires the corresponding signal to function
in a similar fashion. Thus, if a DTE device asserts RTS, it
will probably require CTS to be asserted appropriately by the
connected DCE host/modem/device.

3. A general null-modem configuration, with good functionality
is given here:


FIGURE DTE connected to DTE
(Full function Null modem cable)

(DTE) (DTE)
PIN PIN
NOTE--> (terms relative to righthand DTE)
1----------frame ground-----------1
2---->\/----transmit data<--------2 (2 & 3 reversed in cable)
3<----/\----receive data-------->3
4---->\/----request to send<------4 (4 & 5 reversed in cable)
5<----/\----clear to send------>5
6<----\/----data set ready------->6 (6 & 20 reversed in cable)
7-----------signal ground---------7
8--received line signal detected--8 (aka DCD and CD)
20---->/\----data terminal ready<--20

Notes:"reversed" means that pin 2 on the left-side connector
has been re-wired to be pin 3 on the right-side
connector (and the same for pin 4 and 5, pin 6 and 20).

The DCE to DCE connection is the same except that all
the arrows are reversed in a logical sense.

In the DTE to DTE configuration, sometimes the CD
(Carrier Detect, pin 8) should be tied to one
DTE's DSR (Data Set Ready, pin 6) input. Be careful
to not tie (electrically direct connect) the pin to
both devices' DSR. This note is saying that one
wire is pinned from one DTE's pin 20 to the other
DTE's pin 6, and that wire is directly connected
also to the wire which connects the CD (Carrier
Detect, pin 8) between the devices.

4. A major exposure for equipment in customizing cables is
connecting positive signals to negative signals. This point
elaborates somewhat on the use of positive and negative
signals in RS232 connections.

A device may present one of three signals on a wire, expecting
another device to detect the signal. The signal may be:
a. OFF: a voltage ranging from greater than -3 volts to
less +3 volts
b. MARK: a voltage of -3 or less volts
c. SPACE: a voltage of +3 or more volts

(voltage is measured relative to pin 7: signal ground.)

Any apparent cross-wiring or pin connections that direct tie
together a possible MARK to a possible SPACE could easily
ruin the associated equipment.

Any apparent cross-wiring or pin connections that direct tie
together a possible MARK OR a possible SPACE to signal ground
or frame ground could easily ruin the associated equipment by
acting as a direct short.