This is a 144-bit full-size card. A Programmable Peripheral Interface chip, type 8255-5 provides three 8-bit ports (A, B, and C) for each 24-bit group. Each port can be independently programmed for either input or output. Port C can also be configured as two 4-bit ports. These cards fully implement Mode 0 of the 8255 PPI chip. (Consult the factory if you desire to operate in Mode 1.) Type LS245 bi-directional transceivers sink 24 mA (optionally 64 mA) and source 15 mA. These transceiver buffers provide hysteresis correction on inputs and added drive capability on outputs. The direction of the transceivers is set by software.
I/O connections are via 50-wire interface cables that extend through rectangular cut-outs in the mounting bracket and plug into headers on the card. Strain relief is provided for these cables. Alternate wires are connected to ground to provide shielding and minimize crosstalk.
The I/O buffers can be enabled/disabled under program control. At each 24-bit port, two of the I/O lines can be used for User Interrupts; one line to enable/disable this function and one for the external interrupt signal. Interrupt levels 2-7, 10-12, 14, and 15 can be jumper selected on the IOD-144.
Card I/O bus address selection is by DIP switches on the cards. The address can be anywhere in I/O space. On power-up or reset, all ports are automatically configured as inputs. This prevents spurious outputs. Before use, of course, the PPI should be software configured by writing to its control register. The table on the previous page defines bit assignments at each PPI.