HP97 The journey begins

[teenix]

Hi all,

One of those nights where your brain is not really sleeping, but mulling over a niggly problem that you should have seen already.

I should have looked a bit deeper into how the PIK chip detects an Out Of Paper (OOP) situation. It is not as simple as the home switch setup, even though both methods just ground a switch terminal.

The OOP switch shorts the base of transistor Q11 to ground forcing it to turn off and thus stops the motor moving forward. This ground is also connected to the PIK chip FWD drive output pin.

Still more complicated, in that this OOP setup is bypassed when the Paper Advance button is pressed so that the motor can work to restore a new paper roll.

There must be some trickery inside the chip to be able to drive the motor transistor but not be affected by the direct short to ground - maybe just a resistive setup.

The test for the OOP condition is a PIK1220 instruction. This can execute from ROM both when the printer is idle and when it is printing. While printing however, the FWD pin has a Pulse Width Modulated signal coming out to control the motor speed. There must be some other trickery to detect the OOP condition when this is happening also.

A couple of other things are happening too with the printer which I thought were controlled by the ROM code, but is not the case.

More thinking required....??

cheers

Tony