|Re: my HP-67 died displays "-0.____0-"|
Message #6 Posted by Tony Duell on 26 Apr 2005, 6:30 a.m.,
in response to message #5 by DaveG
That DATA line does sound wrong, I am pretty sure you should see decent pulses there all the time the machine is running.
The ACT bus lines are basically as follows. There are 2 clocks, Phi_1 and Phi_2, and sync which synchronises all chips so that they start the (56 bit) cycle at the same time. Note that sync is not output in _every_ bus cycle, so don't worry if it sometimes seems to be missing.
ISA is the instruction and address line to the ROM. The ACT outputs an address, then reads back an instruction, bit serially, of course. Data is the data line to/from RAMs, etc. Normally it's an output from the ACT, during read cycles it becomes an input and the RAM drives the line. The RAM (or other chip) decodes the instruction sent from the ROM over the ISA line to determine when it should do this, and indeed when it should write data from the DATA line to a RAM location.
AFAIK there are no passive components connected to the DATA line. I can check the schematics later if need be (as ever these are on the HPCC schematics CD-ROM...)
Can you detect a measurable short from the DATA line to ground? If so, you could try desoldering the 8 pin ROM/RAM chips to find out which one is shorted.
Do I take it from the fact that you gave the price of a replacement 67 in pounds that you're in the UK? If so, any way you could get to an HPCC meeting in London? I could take a quick look at the machine for you there.