Information Technology Reference
In-Depth Information
Table 14.1
Main serial standards
EIA
RS-232-C
RS-423-A
RS-422-A
RS-485
ITU
V.28
V.10/X.26
V.11/X.27
Data rate
20 kbps
300 kbps
10 Mbps
10 Mbps
Max distance
15 m
1200 m
1200 m
1200 m
Type
Unbalanced
Unbalanced dif-
ferential
Balanced differ-
ential
Balanced differ-
ential
Number of drivers
and
receivers
1 driver
1 receiver
1 driver
10 receivers
1 driver
10 receivers
32 drivers
32 receivers
Driver voltages
±
15 V
±
6 V
±
5 V
±
5 V
Number of con-
ductors per signal
1
2
2
2
common
common ground
Figure 14.3
Unbalanced digital interface circuit (RS-423)
Noise
RX+
TX+
Termination
resistance
TX-
RX-
Noise
Figure 14.4
Balanced digital interface circuit (RS-422)
14.4 RS-232/485 converter
RS-232 is a standard port on many systems, including PCs and many instruments. A com-
mon requirement is to convert from RS-232 to RS-485, as this allows for long transmission
lengths. If a computer connects to external equipment, it is important to isolate the grounds.
