|Re: HP 85A is UNHAPPY!! Dead CPU?|
Message #2 Posted by Ellis Easley on 8 Sept 2001, 4:24 p.m.,
in response to message #1 by Chris Hoaglin
I have a service manual with a disclaimer saying HP didn't support component level repair and the schematics are reference diagrams, representative of all revisions of the PCB assemblies. The manual does support swapping of socketed ICs, however. 1MA8-0101 (U1) is called the I/O Buffer. It is connected directly to the I/O ports. A common bus connects it to the CRT assembly, the tape drive, the printer/power supply assembly, the CPU (1MB1-0001,U2), the Keyboard Controller (1MB2-0001,U3) and the RAM Controller (1MA2-0002,U8). The other chips on the "logic PC" assembly are RAMs and ROMs.
There is a four phase system clock. Each clock signal is a positive going pulse from 0 to 12 V. There is a timing diagram on the schematic which indicates that each clock has a high pulse width of 0.2 usec and that the leading edge of each clock is delayed 0.4 usec from the leading edge of the previous one. But then it shows the low time of the first clock as 1.6 usec. I think this might be a mistake, and that the period is 1.6 usec. The S/M never states the clock frequency, but I have read elswhere that it is 613 kHz, which would be closer to 1.6 usec than 1.8.
The four system clock signals are called Phase 1, Phase 12, Phase 2 and Phase 21. There are three other clock signals: RCL1, RCL2 and VCL which are used only by the CRT controller. All the clock signals are generated on the CRT assembly. A timing diagram on the CRT schematic shows the period of the clocks as 1.6 usec. They are divided down from a 9.808 MHz crystal oscillator. This divided by 16 is exactly 613 kHz.
Phase 1, Phase 12, Phase 2 and Phase 21 go to the following pins on the following chips:
U1(I/O Buffer): 27, 8, 32, 26 - U2(CPU): 6, 5, 7, 4 - U3(Keyboard Controller): 17, nc, 18, nc - U4,5,6,7(ROMs): 9, 11, 8, 10 - U8(RAM Controller): 23, 25, 24, nc
The four system clocks also go to the I/O connector (byassing the I/O buffer), 1 and 2 go to the tape controller, and 1, 2 and 21 to to the printer controller.
I hope this helps.