User manual

Page 5-5
Manual MPCI-DA12-16.Dh
The circuit is initialized by the computer Reset command (all ports set for input and all buffers
enabled). Both the 8255 control register and the buffer direction latch are accessed at the same
address.
The 8255 control register will latch a new configuration byte when it's written to with bit D7 high.
If, for example, hex 80 is sent to Base Address+23, the group 0 PPI will be configured in mode 0 with
ports A, B, and C as outputs.
At the same time, data bit D7 is also latched in the buffer controller. A high state puts the buffers in
the tristate mode; i.e., disabled. Now, if any of the ports are to be set as outputs, you may set the
values of the respective port with the outputs still in tristate condition. Lastly, to enable the ports a
control byte of hex 00 must be sent to Base Address+23.
Note
All data bits except D7 must be the same for the two control bytes. Those buffers will now remain
enabled until another control byte with data bit D7 high is sent to Base Address+3.