Section Five; Programming The Gpib-Com; The Gpib-Com Registers; Table 5-1. Gpib-Com Registers - National Instruments GPIB-COM User Manual

Section Five

Programming the GPIB-COM

This section presents a description of the GPIB-COM Serial Port Emulator registers and
information on programming the GPIB-COM. You need to use this section only if you are
writing your own serial port device driver.

The GPIB-COM Registers

IBM's serial adapter is a plug-in card for the PC that handles RS-232 communication. The main
component of the adapter is an INS8250 Asynchronous Communications chip or its equivalent,
which is controlled by programming a set of registers on the I/O channel. The names of these
registers and their I/O addresses are given in Table 5-1. Each register is located at a certain offset
from the base address of the board. The addresses are given with an X for the first digit and a Y
for the second digit, where XY=3F for COM1 (base address 3F8), XY=2F for COM2 (base
address 2F8), XY=3E for COM3 (base address 3E8), and XY=2E for COM4 (base address
2E8). Notice that sometimes two registers share the same I/O address. The DLAB bit (bit 7 of
the Line Control Register) determines which register will respond when these addresses are
accessed.
Note: Throughout this section, the term set will be used to mean that a bit is a logical 1, and clear
will mean that the bit is a logical 0.
Register
Transmitter Holding Register
Receive Buffer Register
Divisor Latch LSB Register
Divisor Latch MSB Register
Interrupt Enable Register
Interrupt Identification Register
Line Control Register
Modem Control Register
Line Status Register
Modem Status Register
The following pages describe the functions of each register on the serial adapter and on the GPIB-
COM. For a more complete description of the serial adapter, refer to the IBM Options and
Adapters Manual.
© National Instruments Corporation

Table 5-1. GPIB-COM Registers

Type XY=3F for COM1
XY=2F for COM2
XY=3E for COM3
XY=2E for COM4
write
read
read/write
read/write
read/write
read
read/write
read/write
read
read
5-1
I/O Address
XY8 (DLAB=0)
XY8 (DLAB=0)
XY8 (DLAB=1)
XY9 (DLAB=1)
XY9 (DLAB=0)
XYA
XYB
XYC
XYD
XYE
GPIB-COM User Manual
Offset from
Base Address
0
0
0
1
1
2
3
4
5
6