Hi Jan:

Thanks for jumping in.

And as you suspected the declaration did have something to do with it. I discovered that if I hard coded the physical addresses within my write_output code everything worked as expected...

/* addressing based from cards address */
if (port < 3) then
valveaddr = (0c200h) + port;
else
valveaddr = (0c300h) + (port MOD 3);

This showed that my arithmetic was ok but oddly something else was going on here. The problem was that my PL/M based pointer declared in my set_output procedure was not based on auxiliary memory as in the declaration you pointed out. The code fix was within set_output by declaring ptr as follows...

declare valve based ptr byte auxiliary;

Since in PL/M all memory declarations default to Main the ptr reference in set_output was incorrect. Live and learn. Can't blame PL/M. ;-)