Modem Control Register - National Instruments GPIB-COM User Manual

Programming the GPIB-COM

Modem Control Register

Offset from Base I/O Address = 4
Register Address = XFC
7 6
0
0
The Modem Control Register is a read/write register that controls RS-232 output lines for
communication with a modem or modem emulator. The function of each bit in this register is
explained below.
Bit Mnemonic
7-5r/w 0
4r/w LOOP
3r/w OUT2
2r/w OUT1
1r/w RTS
GPIB-COM User Manual
5 4
0 LOOP
OUT2
Description
Reserved Bits 7 through 5
These bits always read as 0.
Loopback Diagnostic Test Bit
When set, this bit internally connects the outputs of the INS8250
back to the inputs to allow the chip to be tested by writing data to
itself and reading it back for verification.
This bit is ignored by the GPIB-COM circuitry.
Auxiliary Output 2 Bit
The OUT2 serial port output is the inverse of this bit. All
interrupts from the serial adapter are disabled when this bit is
cleared.
When this bit is cleared, interrupts are disabled on the GPIB-
COM.
Auxiliary Output 1 Bit
The OUT1 serial port output is the inverse of this bit.
This bit is ignored by the GPIB-COM circuitry.
Request to Send Bit
The RTS serial port output is the inverse of this bit. When this
bit is set, it indicates that the Controller is ready to send data.
On the GPIB-COM, this bit is used along with the DTR bit to
control the CTS bit of the Modem Status Register.
5-10
3 2
OUT1 RTS
© National Instruments Corporation
Section Five
1
0
R/W
DTR