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RS-422 interface


RS422 is a balanced serial interface for the transmission of digital data. The advantage of a balanced signal is the greater immunity to noise. The EIA describes RS422 as a DTE to DCE interface for point-to-point connections.

37 pin D-SUB male
37 pin D-SUB male connector  at the DTE (Computer)

RS422 was designed for greater distances and higher Baud rates than RS232. In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be used to form an "RS232 extension cord." Data rates of up to 100K bits / second and distances up to 4000 Ft. can be accommodated with RS422. RS422 is also specified for multi-drop (party-line) applications where only one driver is connected to, and transmits on, a "bus" of up to 10 receivers.

Both RS-422 and RS-485 use a twisted-pair wire (i.e. 2 wires) for each signal. They both use the same differential drive with identical voltage swings: 0 to +5V. The main difference between RS-422 and RS-485 is that while RS-422 is strictly for point-to-point communications (and the driver is always enabled), RS-485 can be used for multidrop systems.

Since the basic differential receivers of RS-423-A and RS422-A are electrically identical, it is possible to interconnect an equipment using RS423-A receivers and generators on one side of the interface with an equipment using RS422-A generators and receivers on the other side of the interface, if the leads of the receivers and generators are properly configured to accommodate such an arrangement and the cable is not terminated.

Pin Name Dir Description
1GND---Shield Ground
2SRI<--Signal Rate Indicator
4SD-->Send Data
5ST-->Send Timing
6RD<--Receive Data
7RTS-->Request To Send
8RR<--Receiver Ready
9CTS<--Clear To Send
10LL-->Local Loopback
11DM<--Data Modem
12TR-->Terminal Ready
13RR<--Receiver Ready
14RL-->Remote Loopback
15IC<--Incoming Call
16SF/SR-->Select Frequency/Select Rate
17TT-->Terminal Timing
18TM<--Test Mode
20RC---Receive Twister-Pair Common
21GND---Spare Twister-Pair Return
22/SD---Send Data TPR
23GND---Send Timing TPR
24GND---Receive Timing TPR
25/RS---Request To Send TPR
26/RT---Receive Timing TPR
27/CS---Clear To Send TPR
28IS<--Terminal In Service
29/DM---Data Mode TPR
30/TR---Terminal Ready TPR
31/RR---Receiver TPR
32SS-->Select Standby
33SQ<--Signal Quality
34NS-->New Signal
35/TT---Terminal Timing TPR
36SB<--Standby Indicator
37SC---Send Twister Pair Common
RS422 is a balanced serial interface for the transmission of digital data. The advantage of a balanced signal is the greater immunity to noise. The EIA describes RS422 as a DTE to DCE interface for point-to-point connections.

Technical Description
The data is coded as a differential voltage between the wires. The wires are named A (negative) and B (positive). When B >A then the output is a mark (1 or off) and when A >B then it is counted as a space (0 or on).
In general a mark is +1 Vdc for the A line and +4 Vdc for the B line.
A space is +1 Vdc for the B line and +4Vdc for the A line.
At the transmitter end the voltage difference should not be less than 1.5 Vdc and not exceed 5 Vdc.
At the receiver end the voltage difference should not be less than 0.2 Vdc. The minimum voltage level is -7 Vdc and maximum +12 Vdc.

Max. Distance @ Rate1200 meter/ 4000 feet @ max. 100 kbps
Max. Rate @ Distance10 Mbps @ 12 meter/ 50 ft
Driver Output Resistance100 ohm
Receiver Input Resistance4 kohm min.
Max. Output Current150 mA

The interface is normally terminated on a 4 wire screw block or on a Sub-D37 (RS449), but other interfaces are not uncommon.

RS449 gets it"s blazing speed from the fact that, unlike RS232 which uses signals with reference to ground, it"s receivers look for the difference between two wires. Now the secret, by twisting these two wires any stray noise picked up on one wire will be picked up on the other, because both wires pick up the same noise the differential just shifts in voltage level with reference to ground, but does not change with respect to each other. Remember the receivers are only looking at the difference in voltage level of each wire to the other not to ground. This is what makes all the new wire interfaces work, V.35, RS530, 10baseT, etc.

The biggest problem faced is how the cables are made

The differential signals for RS449 are labeled as either "A and B" or "+ and -". In the case of RS449 wire A or + does not connect to B or -. Wire A always connects to A and B connects to B or + to + and - to -. If you do cross the wires you just inverted the data or clock in your interface. I have never seen any piece of equipment damaged from this, but they don"t work this way either.

Some specs:

Total Number of Drivers and Receivers on One Line1 DRIVER
Maximum Cable Length4000 FT.4000 FT.
Maximum Data Rate100kb/s10Mb/s
Maximum Driver Output Voltage+/-6V-0.25V to +6V
Driver Output Signal Level (Loaded Min.)Loaded+/-3.6V+/-2.0V
Driver Output Signal Level (Unloaded Max)Unloaded+/-6V+/-6V
Driver Load Impedance (Ohms)>450100
Max. Driver Current in High Z StatePower OnN/AN/A
Max. Driver Current in High Z StatePower Off+/-100uA+/-100uA
Slew Rate (Max.)AdjustableN/A
Receiver Input Voltage Range+/-12V-10V to +10V
Receiver Input Sensitivity+/-200mV+/-200mV
Receiver Input Resistance (Ohms)4k min.4k min.

Note: Direction is DTE (Computer) relative DCE (Modem).

37 pin D-SUB female
37 pin D-SUB female connector  at the DCE (Modem)

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