Timex 2048 only 16k

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Paul
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Re: Timex 2048 only 16k

Post by Paul »

landex wrote: Tue May 31, 2022 8:45 pm Hi Paul.
I try it
From 32760 to 32767 it's everything ok. But from 32768 to32780 it adds 1 value at 0, 85 and 170. The 255 it's in every memory fine.

32767 it's the last memory of the 16kb, isn't?
Yes, 32768 is the first address after 16k.
Did I get you right that the results were
1 for 0, 86 for 85, 171 for 170 and 255 for 255?

Please repeat for Addresses 35000 and 44000 because it's weird that you had 3 for 0, 85, 170 and 255.
In theory, there is no difference between theory and practice. But, in practice, there is.
landex
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Re: Timex 2048 only 16k

Post by landex »

Paul wrote: Tue May 31, 2022 10:50 pm
landex wrote: Tue May 31, 2022 8:45 pm Hi Paul.
I try it
From 32760 to 32767 it's everything ok. But from 32768 to32780 it adds 1 value at 0, 85 and 170. The 255 it's in every memory fine.

32767 it's the last memory of the 16kb, isn't?
Yes, 32768 is the first address after 16k.
Did I get you right that the results were
1 for 0, 86 for 85, 171 for 170 and 255 for 255?

Yes.
I will repeat for Adresses 34000 and 35000 tomorrow, because I have to write the program all again. I don't have tape recorder. I will make it on a emulator and save has tap.
Thanks again, Paul
landex
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Re: Timex 2048 only 16k

Post by landex »

Paul, I put the addresses 34000 and 35000.
85 and 255 are ok. 1 and 170 adds 1 value.
Then I repeat with 32768 to 32780, and the value 85 gets 85.

I must made a mistake before, maybe I wrote 80 instead of 85.
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1024MAK
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Re: Timex 2048 only 16k

Post by 1024MAK »

The point of my last test program, is that it writes a value to RAM and then reads it back.
If you work out the values in binary, you can tell which bits in the byte are wrong in the returned value.

DRAM chips of that time typically only handle one data bit per address or the later chips handle four data bits per address.

For example a 4116 DRAM chip is a 1 bit x 16384 (16k) chip. For an eight bit processor like the Z80, to make 16k bytes you need eight 4116 DRAM chips. Each handles one bit of the byte. If one of these DRAM chips fails, this normally affects all 16k bytes of this memory.

The 4416 DRAM chip is a 4 x 16384 (16k) chip. Hence you need two for a Z80 system to make 16k bytes of memory.

From the photos:
MN4264-12 these are 4 x 16384, two are needed to make 16k bytes of RAM.

uPD41464C-12 these are 4 x 65536, two are needed to make 64k bytes of RAM.
There are also some Texas Instruments chips, the part number starts TMS but I can’t read the rest of the part number :(

If you know or can find out which chip stores which bits, combined with knowing which bits are returned with the wrong value, you can work out which DRAM chip(s) are faulty.

Mark
landex
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Re: Timex 2048 only 16k

Post by landex »

Mark, I can't find any TMS chip. Where is he?
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.
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1024MAK
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Re: Timex 2048 only 16k

Post by 1024MAK »

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
MN4264
TMS4416
TMS4416
uPD41464
uPD41464

Mark
sP1d3r
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Re: Timex 2048 only 16k

Post by sP1d3r »

landex wrote: Thu Jun 02, 2022 1:57 am Above the U11 dram, the 3 resistors, the last one has another one above it.
The rightmost resistor of those 3 on your motherboard has a diode soldered to it, I also noticed a component behind the power switch close to the led that looks very dark, as if it's burnt out. On my motherboard it's a resistor and it's normal looking.
landex
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Re: Timex 2048 only 16k

Post by landex »

Thanks mark and sP1d3r!

I am afraid to make something wrong, because my hands shake a bit. :(

I found in the www a schematic of TC2048 rev.5.1.3.

Image
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1024MAK
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Re: Timex 2048 only 16k

Post by 1024MAK »

See also this post ;-)

Mark
landex
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Re: Timex 2048 only 16k

Post by landex »

1024MAK wrote: Fri Jun 03, 2022 3:34 pm See also this post ;-)

Mark
I see! :D
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