National Instruments PC-DIO-24 - Manual (Page 33)

Chapter 4 Register-Level Programming
© National Instruments Corporation 4-3 PC-DIO-24 User Manual
Register Map
The following table lists the address map for the PC-DIO-24. The registers PORTA, PORTB,
PORTC, and CNFG are 8-bit registers in the 82C55A.
Table 4-1. PC-DIO-24 Address Map
Register Offset Address
(Hex)
Size Type
PORTA
PORTB
PORTC
CNFG 0x00
0x01
0x02
0x03 8-bit
8-bit
8-bit
8-bit Read-and-write
Read-and-write
Read-and-write
Write-Only
Note: A number preceded by 0x is a hexadecimal number.
Register Descriptions
Figure 4-1 shows the two control-word formats used to completely program the 82C55A. The
Control Word Flag determines which control-word format is being programmed. When the
Control Word Flag is 1, bits 0 through 6 determine the I/O characteristics of the 82C55A ports
and the mode in which they are operating (that is, mode 0, mode 1, or mode 2). When the
Control Word Flag is 0, bits 3 through 0 determine the bit set/reset format of port C.
Warning: During programming, note that each time a port is configured, output ports A
and C are reset to 0, and output port B is undefined.