IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

Voltage Interchange Information
Interface
Interchange Voltage Binary State Signal Condition Control Function
Positive Voltage = Binary (0) = Spacing =On
Negative Voltage = Binary (1) = Marking =Off
Invalid Levels
+15V----------­
On Function
+3V ----------­
0\1 Invalid Levels
-3V ----------­
Off Function
-15V----------­
Invalid Levels
The signal will be considered in the "marking" condition when the
voltage on the interchange circuit, measured at the interface point, is
more negative than minus three volts with respect to signal ground. The
signal will be considered in the "spacing" condition when the voltage is
more positive than plus three volts with respect to signal ground. The
region between plus three volts and minus three volts is defined as the
transition region, will be considered in invalid levels. The voltage which
is more negative than -15V or more positive than +15V will be
considered in invalid levels.
During the transmission of data, the "marking" condition will be used
to denote the binary state "one" and "spacing" condition will be used to
denote the binary state "zero".
For interface control circuits, the function is "on" when the voltage
is more positive than +3V with respect to signal ground and is "off"
when the voltage is more negative than -3V with respect to signal
ground.
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