Specifications

Appendix A - EIA Specification Summary RS-422 And RS-485 Applications Ebook
Page 87 of 137 Manual Documentation Number: <DocNumber>
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EIA-423 Specifications
EIA RS-423 Specification Summary
Parameter
Conditions
Min
Max
Units
Driver Output Voltage Open Circuit
4
-4
6
-6
V
V
Driver Output Voltage Loaded
RL = 450 ohm
3.6
6
V
Driver Output Resistance
Vo = -2V to 2V
50
ohm
Driver Output Short-Circuit Current
±150
mA
Driver Output Rise and Fall Time
Baud Rate less than
1K Baud
Baud Rate greater
than 1K Baud
300
30
us
% Unit Interval
Receiver Sensitivity
Vcm = -7V to +7v
±200
mV
Receiver Input Resistance
4000
ohm
Comparison of RS-232, RS-423, RS-422 and RS-485
Specifications
The main difference by RS-422 and RS-485 systems is the ability of
RS-485 to tri-state its outputs, enabling multiple drivers to be used in a
multidrop network configuration. However, there are other subtle
differences between the standards.
Another significant difference is the number of receivers that can be
driven on a communications line. The EIA-422 standard specifies that
one RS-422 driver can drive up to 10 RS-422 receivers. The EIA-485
standard specifies that one RS-485 driver can drive up to 32 receivers.
(Up to 32 RS-485 drivers can be connected to the communications line
as long as only one is enabled at a time.)
Note: Some manufacturers' devices are designed to present fractional unit loads (UL) to the
network. For example, some devices are rated as 1/4 UL meaning four receivers add the
same load as one standard device. This increases the effective maximum possible number
of devices on a network significantly.
The following table provides additional specifications for RS-232, RS-
422 and RS-485: