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Communications signal voltage levels
Table 88. Communications signal voltage levels
Interface type OFF voltage level ON voltage level
V. 2 4
EIA-232
X.21bis
Less than -3 volts More than +3 volts
V.35 When terminated by a 100-ohm resistive
load, the differential voltage (A-B) is 0.55
volts +/- 20%.
When terminated by a 100-ohm resistive
load, the differential voltage (B-A) is 0.55
volts +/- 20%.
Note: See the V.35 cable to find out which pin numbers are A and B.
X.21 The differential voltage (A-B) is less than
-0.3 volts.
When terminated by a 100 ohm resistive
load, the differential voltage (B-A) is the
larger of 2.0 volts or 50% of the open
circuit voltage. The open circuit voltage is
between 2.0 and 6.0 volts.
The differential voltage (A-B) is more than
+ 0.3 volts.
When terminated by a 100 ohm resistive
load, the differential voltage (A-B) is the
larger of 2.0 volts or 50% of the open
circuit voltage. The open circuit voltage is
between 2.0 and 6.0 volts.
Note: See X.21 cable diagram to find out which pin numbers are A and B.
ISDN Logic 0
The differential voltage (A-B) is less than
+.75 volts and more than -.75 volts
(nominal).
Logic 1
The differential voltage (A-B) is 0.0 volts
(nominal).
Chapter 9. System Architecture and Configuration 267