RS422 (EIA-422) - RS499 Serial 37 pin

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37 PIN D-SUB MALE 37 PIN D-SUB MALE at the DTE (Computer)



Pin Name Dir Description
1 GND - Shield Ground
2 SRI IN Signal Rate Indicator
3 n/c - Spare
4 SD OUT Send Data
5 ST OUT Send Timing
6 RD IN Receive Data
7 RTS OUT Request To Send
8 RR IN Receiver Ready
9 CTS IN Clear To Send
10 LL OUT Local Loopback
11 DM IN Data Modem
12 TR OUT Terminal Ready
13 RR IN Receiver Ready
14 RL OUT Remote Loopback
15 IC IN Incoming Call
16 SF/SR OUT Select Frequency/Select Rate
17 TT OUT Terminal Timing
18 TM IN Test Mode
19 GND - Ground
20 RC - Receive Twister-Pair Common
21 GND - Spare Twister-Pair Return
22 /SD - Send Data TPR
23 GND - Send Timing TPR
24 GND - Receive Timing TPR
25 /RS - Request To Send TPR
26 /RT - Receive Timing TPR
27 /CS - Clear To Send TPR
28 IS IN Terminal In Service
29 /DM - Data Mode TPR
30 /TR - Terminal Ready TPR
31 /RR - Receiver TPR
32 SS OUT Select Standby
33 SQ IN Signal Quality
34 NS OUT New Signal
35 /TT - Terminal Timing TPR
36 SB IN Standby Indicator
37 SC - Send Twister Pair Common


  • Direction is DTE (Computer) relative DCE (Modem)
  • 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.

Speed and Distance

(Max Recommended)

Terminated Circuits

  • 10 MHz 10 Meters
  • 6 MHz 17 Meters
  • 2 MHz 40 Meters
  • 1 MHz 100 Meters
  • 100 KHz 1000 Meters
  • 10 KHz 1000 Meters

Non- Terminated Circuits

  • 1 MHz 10 Meters
  • 100 KHz 100 Meters
  • 56 KHz 110 Meters
  • 10 KHz 1000 Meters

Cable Design

The design of your cable depends on what you are connecting together and the interfaces involved. There are two standard interfaces types “Data Terminal Equipment” (DTE) and “Data Communication Equipment” (DCE). At this point we will assume that both interfaces are RS449.

DTE to DCE (differential pairs must be twisted)

How to Build a RS422/RS449 Interface

RS449 (RS422) Differential Driver


The resistors Za and Zb are optional. I recommend 10 ohms to bring the interface to 50 ohms and to provide some protection from EMF. Here is a Tip: Note that the A or + signal is on the inverted output pin of the driver, most designers get this switched in their first design. This happens because some data books call the positive pin A and the negative pin B. The RS449 (RS422) A or + must be the inverted pin. This is also true of the receivers.

RS449 (RS422) Differential Receiver


The resistor Zt is optional. I recommend 150 ohms to reduce reflectance. However If you are trying to build a non intrusive receiver I would leave this out. Note this interface when left floating will have an unpredictable output. Some engineers place pull up (to pin A) and pull down (to pin B) resistors of 10K to provide a know state when the cable is unplugged or connected equipment is turned off.

V.24 Driver


V.24 Receiver



Category:Serial Connectors


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