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Communications signal voltage levels
Communications signal voltage levels for several interface types, including V.24 EIA-232 X.21bis, V.35, X21, and ISDN.
| Interface type | OFF voltage level | ON voltage level |
|---|---|---|
| V.24 |
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). |
