Thanks Katie!

[PANAMATIK]

(10-15-2015 08:40 AM)Harald Wrote:  I seem to have just "repaired" my newly acquried HP 27 by inserting a 20k resistor into the 1.6kHz clock line.....

Congratulations for having a working HP-27 now!

The clock line is actually called SYNC. Katie discovered, that the SYNC line shows 1.6 kHz when the calculator works correctly and 3.2 kHz if it doesn't. This is true, but depends on the instructions executed.

The SYNC line is high for 55 us in each 308 us instruction cycle, except for "go to" instructions. What you see on your scope is the idle loop, when the calculator waits for key pressed, which is a repeatedly loop of a test instruction followed by a go to instruction.

Code:


loop:  if 0 = s key  15     ; is no key pressed ? status bit 15 = 0        
         then go to loop    ; then test again

This generates a SYNC pulse every second instruction cycle, which results in a 1.6 kHz signal. If the calculator fails to stay inside the idle loop, perhaps because of executing permanently nop instructions, the SYNC pulse is given every instruction cycle and results in a 3,2 kHz signal.

The reason why the SYNC signal is not given in "go to" instructions is to prevent peripheral chips like RAMs to interpret the "go to" address as an opcode.

Bernhard