10. Parallel I/O
89
Input Buffer
The input buffer is enabled during read operations to transfer the data from connector
J4 to the internal data bus. If the parallel port bit is configured as input, the data read is
the data driven by an external device.
The input buffer is an inverting device. This means that data read from the parallel port
as a logical 0 is a TTL high at connector J4, and data read from the parallel port as a
logical 1 is a TTL low at connector J4.
PROGRAMMABLE REGISTERS
The 16C50A supports standard and enhanced operating modes. Each mode has a
different set of registers associated with it.
•
The three I/O ports at 78h, 79h, and 7Ah are available through connector J4. Refer
to Appendix B, "
Specifications
," for the connector pin assignments.
•
The three I/O ports at 7Bh, 7Ch, and 7Dh are dedicated to managing functions local
to the ZT 8904. Refer to Chapter 11, "
System Registers
," for additional information.
16C50A Standard Operating Mode
Standard operation provides access to all six PIO ports and limited event sense. It is
selected after a power cycle or reset.
Standard I/O Port Addressing
Address
Register
Read Operation
Write Operation
0078h
Port 0 Data
MOD00-MOD07
MOD00-MOD07
0079h
Port 1 Data
MOD08-MOD15
MOD08-MOD15
007Ah
Port 2 Data
MOD16-MOD23
MOD16-MOD23
007Bh
System Register 0
-----
-----
007Ch
System Register 1
-----
-----
007Dh
System Register 2
-----
-----
007Eh
Reserved
-----
-----
007Fh
Write Inhibit
Status
Control