 |
RS-232/RS-422 serial ports
Field Bulletin
Date: April 28,
1999
Products: SC-3, SC-2
Subject: RS-232/RS-422 serial ports
This document provides a basic explanation of RS-232
and RS-422 serial ports and how they are used on the
SC-3 and SC-2 controllers. The following information
should be used as a guide when connecting external
serial devices to the SC-3 and SC-2 control systems.
It is important to remember that the 'RS' in RS-232
and RS-422 stands for 'recommended standard.' In other
words, the connector size and pin-outs can and will
vary depending on the manufacturer of the device.
The associated pin-outs for these serial ports and
some commonly used cables are listed below.
RS-232
The RS-232 port is typically a 9-pin or 25-pin port.
It is designed around a single transmit and receive
line with additional handshaking lines. Many devices
and programs do not require the handshaking lines
for communication and full functionality. Modems,
on the other hand, do require the handshaking lines
for full functionality. The signal level typically
ranges between ±5 to ±12 volts (referenced to ground)
with lengths of up to 50 feet and data speeds of up
to 20kbps. In practice, RS-232 is much more robust
and can stretch twice as far or run speeds 10 times
as fast.
The RS-232 port is generally designed in either
a DTE or DCE layout. DTE stands for Data Terminal
Equipment (like computers, printers, and terminals).
DCE stands for Data Circuit-Terminating Equipment
(like modems and DSU's). The difference between these
two layouts is that they are essentially opposite
of each other. When a DTE device is connected to a
DCE device, a straight-through cable is used. When
a DTE device is connected to another DTE device, a
null-modem cable must be used. The standard pin-outs
for DCE and DTE type devices are shown in Table 1.
| |
|
DTE |
|
DCE |
| |
|
25-pin |
9-pin |
|
25-pin |
9-pin |
| Shield Ground |
SHIELD |
1 |
|
|
1 |
|
| Transmit Data |
TXD |
2 |
3 |
|
3 |
2 |
| Receive Data |
RXD |
3 |
2 |
|
2 |
3 |
| Request to Send |
RTS |
4 |
7 |
|
5 |
8 |
| Clear to Send |
CTS |
5 |
8 |
|
4 |
7 |
| Data Set Ready |
DSR |
6 |
6 |
|
20 |
4 |
| System Ground |
GND |
7 |
5 |
|
7 |
5 |
| Carrier Detect |
CD |
8 |
1 |
|
22 |
9 |
| Data Terminal Ready |
DTR |
20 |
4 |
|
6 |
6 |
| Ring Indicator |
RI |
22 |
9 |
|
8 |
1 |
Table 1 |
RS-422
The RS-422 port is typically a 9-pin or 37-pin port.
It is designed around balanced transmit and receive
lines. The balanced pairs are known as "A" and "B"
or "-" and "+", respectively. It may also have an
"Enable" signal for disconnected or tristated conditions,
though this is only required with an RS-485 driver.
Signal levels typically range from ±2 to ±6 volts,
though many applications use only the positive range
of 0 to +5 volts. Cable lengths can be in excess of
1000 feet when using twisted pair.
The two types of RS-422 pin-outs commonly found
in broadcast-type environments are the "Sony" standard
and the "SMPTE" standard. The Sony pin-out is usually
found on "controlled" devices like VCR's and BETA
machines. The SMPTE pin-out is found on the controlling
devices. This allows for a straight through 9-pin
cable to be used in many of the connections. These
pin-outs are shown in Table 2.
| Not all connections
fall under this category and therefore reference
MUST be made to each appropriate manual for proper
pin-out and cable design. *** The TX+ must connect
to RX+ and the TX- to the RX- *** |
| 9-pin |
TX- "A" |
TX+ "B" |
GND |
RX- "A" |
RX+ "B" |
| Sony (controlled) |
2 |
7 |
4, 6 |
8 |
3 |
| SMPTE (controller) |
8 |
3 |
4, 6 |
2 |
7 |
Table 2
SC-3 pin-out for RS-232
The RS-232 pin-out incorporated on comm. ports 0-4
of the SC-3 is the same as the 25-pin DTE as shown
in Table 1. Port 0 is reserved as the Diagnostic Port.
SC-3 pin-out for RS-422
The SC-3 operates as both a 'controlled' and a 'controlling'
device depending upon the external devices connected
to each serial port; therefore, the pin-out is unique
as compared to the 'SMPTE' or 'Sony' standards previously
mentioned. The RS-422 pin-outs for the comm. ports
1-4 and 3rd party ports 1-8 are shown in Table 3.
| SC-3 |
TX- "A" |
TX+ "B" |
GND |
RX- "A" |
RX+ "B" |
| 25-pin comm.
port |
11 |
9 |
10, 24 |
25 |
18 |
| 9-pin 3rd party
port |
7 |
2 |
4, 6 |
3 |
8 |
Table 3
Common SC-3/SC-2 cables
Table 4 below shows the proper cable pin-outs for
connecting some of the more common external devices
to Utah Scientific's SC-3 and SC-2 controllers. Please
note that the SYS TERM port of the SC-2 is a 9-pin
DTE whereas the RS-232 is a 9-pin DCE port.
| |
SC-3 |
SC-2 |
|
N-vision / DMC-6001 |
RAS-2E |
Intertech PC |
GVG Horizon |
Probel Madi |
PC |
| 25-pin |
9-pin |
9-pin |
9-pin |
9-pin |
9-pin |
15-pin |
9-pin |
9-pin |
25-pin |
 |
2 |
|
2 |
TO
 |
|
2 |
|
|
|
2 |
3 |
| 3 |
|
3 |
|
3 |
|
|
|
3 |
2 |
| 4 |
|
8 |
|
8 |
|
|
|
8* |
5* |
| 5 |
|
7 |
|
7 |
|
|
|
7* |
4* |
| 6 |
|
4 |
|
4† |
|
|
|
4*† |
20*† |
| 7 |
|
5 |
|
5 |
|
|
|
5 |
7 |
| 8 |
|
9 |
|
4† |
|
|
|
4*† |
20*† |
| 20 |
|
6 |
|
1, 6 |
|
|
|
1, 6* |
6, 8* |
 |
11 |
7 |
2 |
TO
|
8 |
|
4 |
14 |
8 |
|
|
| 9 |
2 |
7 |
3 |
|
3 |
6 |
3 |
|
|
| 10, 24 |
4, 6 |
4, 6 |
4, 6 |
|
5 |
8** |
4, 6 |
|
|
| 25 |
3 |
8 |
2 |
|
1 |
13 |
2 |
|
|
| 18 |
8 |
3 |
7 |
|
2 |
5 |
7 |
|
|
| * Usually
not needed |
† More
than 1 pin connected |
** via 100
ohm in-line resistor |
| |
| Table 4 |
|
