<p>More info. According to the HP white paper on the HP 60/901 ink heads, there are 416 jets per CYM color, and 336 on the black head.</p>
<p>Each of those heads can print at 10M drops/second.</p>
<p>I would expect that the shift register operates on an 10Mhz clock, and the CYM channels get 52 bytes of on/off data, and the K channel gets 42 bytes.</p>
<p>If they were smart about it, the printhead controller would have a couple of Kb shift buffer, and watermark signal to indicate when there is more room in the shift buffer.</p>
<p>That way, the printhead controller could shift-out the jet patterns as it detects an optical encoder signal, and the shift buffer would automatically shift out the bits for that encoder line.</p>
<p>I think this is probably how they do it, since there are only ~5000 marks on the encoder strip – only 600dpi if you use standard quadrature encoding.</p>
<p>However, if the shift-buffer ejects ink on every <b>transistion</b> (white or black of the encoder strip) then 1200dpi is easily achieved.</p>
<p>Given that analysis, I suspect the following signals would be needed:</p>
- One clock signal for all 4 printhead buffers
- One watermark signal for the fullness of the printhead buffer
- One shift data in signal for each printhead buffer
- One shift data out signal for the CYM cart (EEPROM serial)
- One shift data out signal for the K cart (EEPROM serial)
<p>If I did my math right, that should be 8 I/O signals, and would completely cover all the functionality of the printhead controller.</p>
<p>This also can explain the initial head movement – the head moves to the wiper area, the controller sends ‘fire all jets’ to the printheads until the watermark signal occurs, then runs the head back and forth over the wiper area until the watermark clears, then goes to the suction cap area. By the time the head is at the suction cap area, it’s pretty much guaranteed that the shift registers will be completely empty, and ready for the next scan lines.</p>
<p>Now that I know what to look for, it should be pretty easy to use the ‘mask off a signal with tape’ method to determine the function of each of the signal lines.</p>