HP-19C Repair Odysee

[teenix]

(07-06-2021 07:15 AM)PANAMATIK Wrote:  That is correct, but let it more precise:

At power up the ACT reads the initialization instructions from ROM. When it encounters the pik_home? test instruction, the PIK chip executes the instruction and tells the ACT if HOME switch is activated or not. If not , the ACT branches to code which invokes the pik_cr? instruction, which let the PIK start the motor until a debounced negative edge occurs on the HOME input, i.e. HOME position is arrived.

ACT and PIK and ROMs must be OK to perform the correct HOME return movement.

Bernhard

Oh ok, that is interesting, maybe there are different ROM and PIK versions.

Both my 19C and 67 emulators running standard Microcode don't show a PIK HOME? instruction getting executed on start up or during the key wait loop, only - Is there a key press available? - PIK1320.

The carriage would not home itself at switch on when running the standard HP-97 Microcode on my HP-97 CPU board which caused me to do some head scratching. My guess was the PIK chip senses the state of the carriage through it's HOM pin input and homes the carriage by itself - but this only happens at start up.

Once up and running and waiting for something to happen, if a print operation is to take place, then the Home? and Paper_Out? instructions do get executed (setting the STATUS flag [3]) before the printing takes place. If STATUS [3] flag is set, the Microcode shows error on the display and printing is stopped or not started.

Obviously the HOM pin is required on the PIK chip because it detects the head position and stops the motor when the carriage is back home. It wouldn't be wise for the Microcode to do this because it might be doing something else and unable to monitor the situation. For example, the printer HOM instruction is not executed when a program is running in MAN mode.

Under some conditions, (example: printing multiple lines), if the 97 and 19C carriage will not home, the Microcode will stay in an infinite loop.

Code:


HP-97
$38D    0 -> s3
        pik1120    // carriage home? (sets S3=1 if so)
        if s3 = 0
          then go to $38D  // no

HP-19C
$306        PIK1120
        if s3 = 0
          then go to $306

cheers

Tony