landex wrote: ↑Thu Jun 02, 2022 1:57 am
Mark, I can't find any TMS chip. Where is he?
On the board in the photo that sP1d3r posted.
landex wrote: ↑Thu Jun 02, 2022 1:57 am
But I found something that I think could be wrong: Above the U11 dram, the 3 resistors, the last one has another one above it.
That looks like it may be a signal diode. Hard to tell from the photo.
If the computer was working before, it’s not likely to be that though. As that could be a manufacturer modification.
Test values:
0, 85, 170, 255
In binary these are:
Code: Select all
76543210 data bit/binary digit
———————-
00000000 = 0
01010101 = 85
10101010 = 170
11111111 = 255
Your results?
00000001 = 1
10101011 = 171
Which indicates that the DRAM chip that provides data bit/binary digit 0 (D0 on the Z80) is always returning 1 (or logic high).
If there is a circuit schematic available, that will tell you which DRAM chips are connected to D0 (pin 14) on the Z80. Or you can use a digital multimeter on the 200 ohm (or equivalent) range (with the computer disconnected from the power) to work it out. Test between pin 14 on the Z80 and pins 2, 3, 15, 17 of each of the DRAM chips. If the DRAM pin is directly connected to pin 14 on the Z80, the result will be less than 5 ohms. An ‘OL’ or ‘OR’ or ‘1’ in the
left hand digit indicates either open circuit or the resistance is higher than can be shown when on that range. So switch to a higher range. If the result is between 200 ohms and 500 ohms, it could be a ‘bus’ resistor wired in series. Anything higher than 10000 ohms (10k) is a open circuit.
Because the DRAM chips are 16k types, there could be two or three that are all connected to pin 14 on the Z80.
The pin out of the Z80 is
here
The data pins for MN4264, TMS4416, uPD41464 DRAM chips are pins 2, 3, 15, 17.
DRAM chip pin outs:
- MN4264
- TMS4416
- uPD41464
Mark