I noticed someone earlier said the 65C22 /IRQ line wasn't set up to be wire-ORed together with other chips (e.g. incompatible with the NMOS version). So, I went and looked in my Rockwell 65C22 datasheets last night, and it said the /IRQ output line requires a pull-up resistor...

Can someone else verify this?

Toshi

It's quite possible that Rockwell is O.C.
The WDC chip is *not* O.C., according to their data sheet.
To be sure, you can easily test it on a breadboard: connect a resistor, say 20K or so, from the IRQ to ground. If this causes the IRQ to go low, then it's an O.C. output.
Actually, even without the resistor, the output can be expected to look "low" when it's floating.

Pete

The Rockwell R65C22 IRQ line is open drain and can be wire ORed. The Western Design Center (WDC) W65C22S is active High and Low and can not be wire ORed. WDC did this in order to acheive fast operating speeds. I have been useing the W65C22S and it is a really good device.

Gregqci