IBM RT Series Hardware Reference Manual page 67

During the transmission of data, the marking condition denotes the binary state 1 and the spacing
condition denotes the binary state O.
For interface control circuits, the function is on when the voltage is more positive than + 3 V dc with
respect to signal ground and is off when the voltage is more negative than -3 V dc with respect to
signal ground.
Interface
Interchange
Binary
Signal
Control
Voltage State Condition
Function
+3Vdc to +25Vdc
Binary 0 Spacing
=On
-3Vdc to -25Vdc Binary 1 Marking
= Off
Figure 10.
mM
5080 Peripheral Adapter Signal Levels
5080 Peripheral Adapter 11