plotters again

You _will_ lose the digitising functionality if you use the HPIB connector and set the machien to 'listen only' (if you can do that, most HP HPIB plotters do have that feature). In that mode there is no way the plotter can output data. I don't know the 9872 that well. I have soem experience of the internals of the 7245, which from what I've seen has somewhat similar electronics (the motor driver interpolator is almost identical circuitry, for exampl). Now on the 7245 the HPIB interface is very hardware-intensive -- even the HPIB bus commands (serial poll enable/disable, etc) are decoded in hardwre. Of course the handshanke and addressing are handles in hardware too. All the processor does is get a signal that says 'read a byte from the HPIB data register' and the processor does so (and interpets the bytes read as HPGL commands, of course). If the 9872 is like that, it should be very easily possible to work out what the 9815 interface is doing, since presumanbly it's also mostly handled in hardware, possbily even much the same hardware. From what you've said, it must be more than just a listen-only (unaddressable) HPIB interface, since the plotter can send data back to the 9815 -tony