8-Port MIL-STD 188 signal voltage levels
Voltage levels for the MIL-STD 188 Adapter can be explained through normal mark and space polarity or mark and space polarity inversion.
Voltage levels for the MIL-STD 188 Adapter are explained in the following sections:
- Normal Mark and Space Polarity
- Mark and Space Polarity Inversion.
The signal is in the mark state when the voltage on the interchange circuit, measured at the interface point, is less than -4 V dc with respect to the signal ground. The signal is in the space state when the voltage is greater than +4 V dc with respect to the signal ground. The region between +4 V dc and -4 V dc is defined as the transition region and is not a valid level. The voltage that is less than -6 V dc or greater than +6 V dc is also not a valid level.
During the transmission of data, the mark state denotes binary 1 and the space state denotes binary 0.
For interface control circuits, the function is "on" when the voltage is greater than +4 V dc with respect to the signal ground and is "off" when the voltage is less than -4 V dc with respect to the signal ground. MIL-STD 188 signal levels are shown in the following table:
| Interchange Voltage | Binary State | Signal Condition | Interface Control Function |
|---|---|---|---|
| + Voltage | 0 | Space | On |
| - Voltage | 1 | Mark | Off |
Military standard MIL-STD 188 requires that adapters provide the capability to optionally invert the polarities of the mark and space states of the transmit and receive lines. The capability is provided independently on each port.
The DUART modem control register bit 3 (Out 2) is used for this purpose. When bit 3 is set to a value of 1, the polarities for the mark and space states are set to the normal state. When bit 3 is set to a value of 0, the polarities for the mark and space states are inverted.
The signal is in the space state when the voltage is less than -4 V dc with respect to the signal ground. The signal is in the mark state when the voltage is greater than +4 V dc with respect to the signal ground.
The region between +4 V dc and -4 V dc is defined as the transition region and is not a valid level. The voltage that is less than -6 V dc or greater than +6 V dc is also not a valid level.
The electrical characteristics of the 8-Port asynchronous MIL-STD 188 adapter ports conform to those sections of MIL-STD 188-114 that address an unbalanced voltage interface. The standard is dated March 24, 1976.
The adapter ports meet the functional requirements for asynchronous operation (start-stop protocol) as described in the EIA Standard 232C dated October 1969 and in the EIA Standard 232D dated January 1987.