|Re: HP-35 display anomaly|
Message #4 Posted by Eric Smith on 19 July 2004, 9:58 p.m.,
in response to message #3 by David Ramsey
Further experimentation reveals that if the first digit after the decimal point is "2", then the 2 will occupy the same position as the decimal point,
I've looked up the technical details in the patent, and the details of my previous posting in this thread are not completely accurate.
The display is scanned from right to left, one digit per processor word cycle. During each word cycle, one digit each from the A and B registers are used by the ARC to determine what digit and/or decimal to display, decoded into seven segments and the decimal flag, and sent to the anode driver. The signals are sent over five wires, time-multiplexed in a strange fashion. The anode driver decodes them to determine what digits to light, and also decides whether to generate a step pulse to the cathode driver. For the case of a decimal, the anode driver is supposed to generate an extra step pulse after the digit segments have been driven and before the decimal is driven, to prevent them from overlapping as you have seen.
The decimal point indication is carried on the same signal line that carries the F and G segments.
Internally to the ARC, the digit 2 in the B register is used to indicate the decimal position. This is interesting since an actual display of the digit 2 occurs when a 2 is in the A register. So a decimal followed by a 2 would have the 2 digit in BOTH the A and B registers.
I'm not sure exactly what to conclude; it seems likely that the cathode driver is fine and that either the ARC or anode driver are malfunctioning. I don't see how a dirty contact between the boards could have this result, though I suppose it is not impossible. At this point I think I'd bet on the ARC.