RS-422 (EIA-422) serial interface pins and signals

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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, but RS-422 is a multidrop standard, allowing for one driver and up to 10 receptors, and rs485 is a multipoint standard, allowing for up to 32 devices (drivers, receivers or transceptors)

Since the basic 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
1	GND		Shield Ground
2	SRI		Signal Rate Indicator
3	n/c	-	Spare
4	SD		Send Data
5	ST		Send Timing
6	RD		Receive Data
7	RTS		Request To Send
8	RR		Receiver Ready
9	CTS		Clear To Send
10	LL		Local Loopback
11	DM		Data Mode
12	TR		Terminal Ready
13	RR		Receiver Ready
14	RL		Remote Loopback
15	IC		Incoming Call
16	SF/SR		Select Frequency/Select Rate
17	TT		Terminal Timing
18	TM		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	/RD		Receive Data TPR
25	/RS		Request To Send TPR
26	/RT		Receive Timing TPR
27	/CS		Clear To Send TPR
28	IS		Terminal In Service
29	/DM		Data Mode TPR
30	/TR		Terminal Ready TPR
31	/RR		Receiver TPR
32	SS		Select Standby
33	SQ		Signal Quality
34	NS		New Signal
35	/TT		Terminal Timing TPR
36	SB		Standby Indicator
37	SC		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.

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: