HP97 The journey begins

[teenix]

Hi all,

Some more progress.

I have removed the original printer interface circuit board and have my CPU board driving the print motor and print head directly. I set the motor running and tried to slow it down with finger pressure and it didn't want to, and also I made small changes in battery voltage and that did not have any appreciable effect either so I assume the new motor driver is working ok.

I have removed the idler cog so the carriage doesn't actually move during this early testing as I don't want to damage anything. Unlike the 19C printer, this mechanism can be damaged if the carriage moves faster than the print data is being sent out. The print head is also disconnected until I know for sure that the code will not keep a pixel on too long and risk damage.

If the real 97 goes haywire and the printer motor stays energized either forward or backward then the carriage will jam hard against the printer frame. I cannot see any original mechanism to detect this error, so some serious damage may result. The HP-41 printer has an extra microswitch to detect the carriage limit while printing. This might suggest why the original idler cog is made out of some sort of resin instead of metal. Apart from possibly quieter operation, it might also fail (strip teeth) under improper conditions and avoid other damage. The new motor driver has an overcurrent detect so if the motor stalls for some reason the power will be removed.

At this stage the printer is only doing a line feed from a simulated press of the paper advance switch. This all probably doesn't sound much but when you consider what is actually happening to make this occur it is quite a lot.

The print motor is responding to the HP-97 microcode and the instructions being fed to the simulated PIK chip. The motor moves forward, brakes and then reverses to home, brakes and switches off. I have test points that activate in my code to simulate the home switch operating.

Onward...

cheers

Tony