HP Vectra Reference Manual page 147

Modem Control Register (XFCH)
This register controls the modem signals. It also allows the
HP24540A to be set into diagnostic mode. In the diagnostic
mode, transmitted data is received immediately. The receiver and
transmitter interrupts and the modem control interrupts are fully
operational, allowing the interrupts to be tested.
Bit Data Definition
0 1 Data terminal ready (CD) signal active.
0 CD signal inactive.
1 Request to sent (CA) signal active.
0 CA signal inactive.
2 Controls the -- OUT1 signal from the controller chip.
Can be 0 or 1.
3
Controls -- GUT2 signal from the controller chip.
Enables INTRPT generated by the interrupt enable
~
register.
0
The -- OUT2 output is forced inactive.
"-
4 Enables the modem loopback feature (diagnostic
test) as follows:
•
Receiver serial input is disabled.
• Transmitter serial output is set to the active state.
•
The output from the transmitter shift register is
looped back to the receiver shift register.
• The four modem control inputs to the modem
status register are disabled.
• The four modem control outputs from the
modem control register are internally connnected
to the four modem control inputs.
5-7 0 Always.
130 Serial/Parallel Card