RS423 interface

a Tutorial


RS-423 

The RS-232 standard defines a bi-directional interface between exactly two communicators, the RS-423 standard defines a uni-directional interface between one transmitter and many receivers. For example, a single computer may need to update a number of terminals that are displaying, for example, a customers order information at multiple locations throughout a warehouse. The data direction is always in one direction.

RS-423 allows for distances up to 4000 feet but limits data rates to only 100 kb/sec for a maximum of ten receivers. The voltage levels are +3.6 to +6 volts to represent a binary 0 and -3.6 to -6 volts to represent a binary 1. The voltage levels are defined relative to an earth ground potential assumed to be zero volts. Consequently a difference in ground voltage levels will result in the Common Mode Voltage problem that will confuse the data values.

RS-423 is very similar to RS-422.  RS-423 is a serial interface between one DTE and one DCE, unlike RS-422.  A DB25 connector is sometimes used in place of the typical DB37, although the DB37 is still used.  Like RS-422, all signals use both the A and B lines of a pair, but the B lines in RS-423 are all tied to the Ground (GND).  RS-423 is also a single ended signal rather than the balanced signal of RS-422.  Most RS-423 signals are synchronous, but asynchronous signals can be found on the MMJ, RJ, DB9, DB15, and 4-wire screw terminals.

RS423 Interface

RS423 is a serial binary data interchange between a DTE and a DCE. This interface is used for data rates from 20kbps to 10Mbps. This interface is usually found on a 25 or 37 pin "D" style connector. There is an "A" and a "B" line for all interface signals. All the "B" lines are tied to signal ground.

DB25 Unbalanced DB37

1

Shield (A)

1

2

Transmit Data (A)

4

14

Transmit Data (B)

22

3

Receive Data (A)

6

16

Receive Data (B)

24

4

Request to Send (A)

7

19

Request to Send (B)

25

5

Clear to Send (A)

9

13

Clear to Send (B)

27

6

Dataset Ready (A)

11

22

Dataset Ready (B)

29

7

Signal Ground

19

8

Receive Line Signal Detect (A)

13

10

Receive Line Signal Detect (B)

31

17

Receive Timing (A)

8

9

Receive Timing (B)

26

24

External Timing (A)

17

11

External Timing (B)

35

15

Transmit Timing (A)

5

12

Transmit Timing (B)

23

18

Local Loopback

10

20

Data Terminal Ready (A)

12

23

Data Terminal Ready (B)

30

21

Remote Loopback

14

25

Test Mode Indicator

18

 

Signal Rate Indicator

2

 

Incoming Call

15

 

Select Frequency

16

 

Receive Common

20

 

Terminal In Service

28

 

Select Standby

32

 

Signal Quality

33

 

New Signal

34

 

Standby/Indicator

36

 

Send Common

37

Signal and Electrical Characteristics

DC Voltage -4 to -6 +4 to +6
Binary State 1 0
Signal Condition Marking Spacing
Function Off On

RS423 is an unbalanced circuit where all signals on the interface are returned on one signal ground wire (common). The output voltage from a driver are usually + or - 5 volts. The receiver receives a valid signal if the voltage is at least +0.2 or -0.2 volts. The maximum speed is 100kbps and the maximum cable length is dependent on the speed and on the capacitance of the cable. The maximum cable length is 4000 feet/1200m at 1kbps and 50 feet/15m at 100kbps.

Types of Signals

Ground or Common Return Signal return path for all interchange circuits
Data Circuits Serial binary data from DTE to DCE or from DCE to DTE
Control Signals Are signals that control the operation of the DCE or DTE
Timing Circuits Are the clock signals that are passed from a DCE to a DTE or from a DTE to a DCE for the purpose of synchronizing the transmit data or the receive data

The above information is primarily for synchronous types of applications. For the asynchronous applications a wide variety of connector types are used. Also these RS423 devices only use the data circuits, a ground and sometimes one or two control signals.

SPECIFICATIONS RS423
Mode of Operation SINGLE -ENDED
Total Number of Drivers and Receivers on One Line 1 DRIVER 10 RECVR
Maximum Cable Length 4000 FT.
Maximum Data Rate 100kb/s
Maximum Driver Output Voltage +/-6V
Driver Output Signal Level (Loaded Min.) Loaded +/-3.6V
Driver Output Signal Level (Unloaded Max) Unloaded +/-6V
Driver Load Impedance (Ohms) >=450
Max. Driver Current in High Z State Power On N/A
Max. Driver Current in High Z State Power Off +/-100uA
Slew Rate (Max.) Adjustable
Receiver Input Voltage Range +/-12V
Receiver Input Sensitivity +/-200mV
Receiver Input Resistance (Ohms) 4k min.

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