1. RS422, RS485, RS423, RS449 and V.35
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

2. Introduction
Serial interface standards that support longer cable lengths and higher data rate than RS232.
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

3. Agenda Balanced Line Interface RS422 RS485 RS423 RS449 V.35
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

4. Vocabulary
Skew Rate
Measure the slope of signal transitions between Mark and Space
High skew rate causes interference to adjacent wires within the same cable (i.e. crosstalk)
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

5. Balanced (Differential) Line Interface
A pair of wires is used to carry each signal. The data is encoded and decoded as a differential voltage between the two lines
Advantages:
the interface is unaffected by differences in ground voltage between sender and receiver
if lines A and B are close together (e.g. in twisted pairs), they will be affected almost identically by external electromagnetic noise, which can then be eliminated at receiver
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

6. Balanced Line Interface
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

7. RS422 Balanced line interface
Mark: VA – VB > +0.2V
Space: VA – VB < –0.2V
Strictly for point-to-point communications
Recommended maximum cable length is 4,000 ft. at 100kbps
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

8. RS485 = RS422 with tri-state drivers Used in multi-drop networks
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

9. RS423 Single-ended (unbalanced) Mark: –3.6 to –6V Space: +3.6 to +6V
one pair of wires per signal, one of it is GND
crosstalk is further reduced by using lower signal skew rate
Mark: –3.6 to –6V
Space: +3.6 to +6V
Can be used in one-to-many-points communications
Recommended maximum cable length is 40ft. at 100kbps, for 10 receivers
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

10. RS422, RS423 Speed and Length Limits
Source: Beyda, Basic Data Communications, Prentice Hall
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

11. RS449
Replace RS232 in high speed synchronous communication between DTE and DCE
Combine both balanced and unbalanced signals on the same interface
RS422: for fast changing signals like TD, RD and Clock
RS423: for slow changing signals like RTS, CTS, DSR, DTR, … etc.
Use a DB37 connector
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

12. V.35
= V.10 + V.11
Widely used to connect high speed (> 56kbps) DCEs in WAN (e.g. T1, Frame Relay, ATM networks)
Combine both balanced and unbalanced signals on the same interface
RS422: for fast changing signals like TD, RD and Clock
RS232: for slow changing signals like RTS, CTS, DSR, DTR, … etc.
Easily recognized by it bulky 34-pin connector
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

13. V.35 Connector
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

14. V.35 Signals Pin Signal A Chassis Ground B Signal Ground C
Request to Send D
Clear to Send E
Data Set Ready F
Receive Line Signal Detect H
Data Terminal Ready J
Ring Indicator P
Transmitted Data
(Signal A) R
Received Data S
(Signal B) T U
Terminal Timing V
Receive Timing A W X
Receive Timing Y
Transmit Timing AA
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

15. Summary
RS422 and RS423 support longer cable length at higher data rates than RS232
RS422: benefit from balanced line interface
RS423: benefit from individual ground return and lower skew rate
RS449 and V.35 combine both balanced and unbalanced signals on the same interface
RS449: RS422 + RS423
V.35: RS422 + RS232
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35

16. Where to Get More Information
Data Communications (E&T2760): RS422, RS485, RS423, RS449 and V.35