Category : Tutorials + Patches
Archive   : RS232CBL.ZIP
Filename : MDM-CONN.TXT

RS-232 Modem Connections

by Dennis Recla

Nov. 18, 1984


Every type of modem with an RS-232 I/O port has its own specific
requirements of connections to make full use of the modem. Most
common modems are configured as a DCE (Data Communications Eq.)
for connection to a DTE (Data Terminal Eq.) device. The following
table lists the various pins and their identification required by
various modems.


Pin | Type | EIA | Function
------------------------------------------------------
1 | H V | AA | FG Chassis Ground
2 | H V B | BA | TXD Transmitted Data
3 | H V B | BB | RXD Received Data
4 | V | CA | RTS Request to Send
5 | H V B | CB | CTS Clear to Send
6 | H V B | CC | DSR Data Set Ready
7 | H V B | AB | SG Signal Common
8 | H V B | CF | CXD Carrier Detect
9 | V B | +P | +V +12 Volts
10 | V B | -P | -V -12 Volts
11 ----------------- NOT USED
12 | V B | CI | CI Speed indicator
13 ----------------- NOT USED
14 ----------------- NOT USED
15 | V B | DB | SCT Serial Clock Transmit
16 ----------------- NOT USED
17 | V B | DD | SCR Serial Clock Received
18 | V B | CN | CN Busy Out
19 ----------------- NOT USED
20 | H V B | CD | DTR Data Terminal Ready
21 | V B | RL | RDL Remote Digital Loop
22 | H V B | CE | RI Ring Indicator
23 | V B | CH | SS Speed Select
24 | V B | DA | SCTE External Serial Clock TX
25 | V B | CN | TM/CN Test Mode indicator

H= Hayes Smartmodem 300/1200
V= Vadic Modems 3451, 355, 212PA, 212LC
B= Bell 212A Modem


Each pin on the 25 pin "RS-232" connector have the following
discriptions and requirements. Normally the connector on a
Modem is a part type DB-25S, this is a common part number for
a 25 pin connector having sockets instead of pins. The mating
connector would be a DB-25P.

Pin 1 FG (Chassis Ground)- Common return to AC protective
ground.

Pin 2 TXD (Transmitted Data)- Serial data from terminal
equipment to modem transmitter. Data will be
accepted by the modem when the CTS signal is on.
When CTS is OFF, TXD is internally forced to the
MARK condition. MARK = -3 to -25 V; SPACE = +3 to
+25 V.

Pin 3 RXD (Received Data)- Demodulated serial data from modem
receiver to terminal equipment, active when the CXR
signal is ON. Held in MARK condition when CXR is OFF.

Pin 4 RTS (Request To Send)- Not used normally for full duplex
operation, this control signal from the terminal equip-
ment may be optionally tied to CTS for controlled-
carrier operation.

Pin 5 CTS (Clear To Send)- Control signal from modem to
terminal equipment indication that modem is ready to
transmit data. Normally active when CXR signal is on,
CTS is disabled during Digital Loop-back test mode.

Pin 6 DSR (Data Set Ready)- Control signal from modem to
terminal equipment indicating that modem is off hook
(Connected to the TelCo line). DSR may be ON or OFF
in test mode, depending on user options selected.

Pin 7 SG (Signal Common)- Commong ground reference point
for all terminal interface circuits except chassis
ground. May also be connected to chassis ground with
user option.

Pin 8 CXR (Carrier Detect)- Control signal from modem to
terminal equipment indicating that the modem is re-
ceiving valid energy suitable for demodulation. Also
called Receive Line Signal Detector.

Pin 9 +V (+12 V Test)- Positive dc test voltage, 1 kohm,
nominal.

Pin 10 -V (-12 V Test)- Negative dc test voltage, 1 kohm,
nominal.

Pin 11 NOT USED

Pin 12 CI (Speed Indication)- Control signal from modem to
terminal equipment indicating operating speed of mo-
dem. If ON, modem is in high-speed data mode; if OFF,
modem is in low-speed mode.

Pin 13 NOT USED

Pin 14 NOT USED

Pin 15 SCT (Serial Clock Transmit)- Timing signal from modem
to terminal equipment indicating center of each trans-
mitted signal element (data bit) when modem is operat-
ing in high-speed (1200 bps) synchronous mode. Data on
TXD is sampled by modemon each negative (ON to OFF)
transition of SCT. If the Slave Clock Control option
is enabled, SCT is phase-locked to SCR. If the Transmit
Clock Select is set to external, SCT is phase-locked to
SCTE.

Pin 16 NOT USED

Pin 17 SCR (Serial Clock Received)- Timing signal from mo-
dem to terminal equipment indication center of each
received signal element when modem is operating in
high-speed synchronout mode. The negative transition
of SCR specified valid sample point for data on RXD.

Pin 18 CN (Test/Busy Out)- Optional control signal from ter-
minal equipment to modem used to busy out modem and/
or initiate local test mode.

Pin 19 NOT USED

Pin 20 DTR (Data Terminal Ready)- Control signal from ter-
minal equipment to modem indicating that terminal is
ready for data communication. Prepares modem for
(Automatic) Connection to the telephone line and main-
tains connection once established. In automatic answ-
ering application, the modem will connect to line when
the trailing edge of the ring signal (RI) is received
if the signal is ON.

Pin 21 RDL (Remote Digital Lookback)- Optional control sig-
nal from terminal equipment to modem used to initiate
remote test mode.

Pin 22 RI (Ring Indicator)- Control signal from modem to ter-
minal equipment indication that a ring signal has been
detected (i.e., a call is being received).

Pin 23 SS (Speed Select)- Optional control signal from ter-
minal equipment used to select data rate for origin-
ating a call.

Pin 24 SCTE (External Serial Clock Transmit)- Optional timing
signal from terminal equipment to modem that clocks
transmitted data as described previously under SCT.
The clock must have a frequencyof 1200 Hz +/- .01%
with peak distortion of negative transitions no greater
than 0.5% as per RE334.

Pin 25 TM (Test Mode) or CN (Test/Busy Out)- This line may
be configured as a test-mode indicator from the modem
to the terminal, or as a local test control line from
the terminal to the modem, depending on the user
options selected.

These requirements will vary from modem to modem, so it is best to look
closely at the specific pin connections used by your own modem.