handshaking logic

[O. Zimmermann]

Bonjour !

I want to set up handshaked communication from a PCI-6503 (ie. 8255
chip) digital i/o card. It still fails and I have a hard time
understanding precisely how digital i/o vis should be used and
configured. For example, how do I initiate Write and Read actions
using a "bidirectional" group buffer ? None of NI examples
demonstrates the issue of combining read and write actions.

If any of you has gone through handshaking using a 8255 chip, or if
you masterize the operation of dig i/o vis (buffers, groups etc...)
please let me know...


oz

[Bharatinder Sandhu]

Hi:

8255 chip does support bi-directional mode.Two handshaking lines are used, STB (PC4) and ACK (PC6), both active low. Initially, hold both handshaking lines high, port A is in high impedance state.

To write to port A, hold ACK* low and STB* high, use Digital Single Write.vi to write to the port. As soon as you bring the ACK* line high again, port A goes back to high impedance mode, thus the pattern you just wrote will no longer be present.

To use port A as input port, hold ACK* high. Port A is now an input port and you can externally drive the lines of the port without damage, However, you have to "latch in" the inputes from the port to the input buffer by toggling STB*, then read by calling Digital Single Read.vi