HP97 The journey begins
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04-18-2020, 01:55 AM
Post: #220
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RE: HP97 The journey begins
Hi all,
I have been testing the print head drive code in a test jig. It is quite a complex task to produce the data required to drive these heads at the correct time, including giving the type face an italic look as the real printer does. On top of that the output has to be pulse width modulated with a 10KHz signal in accordance with the battery voltage formula that the original printer analog circuitry creates. The italic way of outputting the pixels is to keep the total head drive current progressively applied instead of all at once. The test jig is "printing" 9.999999999 +99 *** Here is an image of part of the print data as seen on my scope. It shows the pixel dots in the upper part with a small section enlarged in the middle part which shows the PWM control for each pixel. Further down is shows the PWM for normal battery voltage and to the right at lower battery voltage. The calculator microcode still has to function plus the "PIK" keyboard scanning and other bits and pieces while all this is happening so it is quite a complicated set of tasks for a single processor. As a safeguard, the PIC will have its watchdog timer enabled in case the code freezes and one or more heads stay energised. Hopefully it will reset before any damage is done. cheers Tony |
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