Resurrecting an MTX500.

About original Memotech hardware.
stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 10 Apr 2019 11:24

I did get one interesting boot last night.

Instead of a continuous tone I got a clicky-buzzing from the sound sub-system. 'Scoping the data bus showed it active and the speed of the buzzing changed (increasing when the probe was touched on one or two of the data lines). It was almost as if the sound chip was acting as a bus monitor.

Could the 74LS244 (E2), which buffers the data lines with the LCDx lines possibly be a cause of data bus corruption I wonder?

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 10 Apr 2019 12:58

Every instruction cycle the Z80 will generate a DRAM refresh address on A0 to A15, the state of A0 to A6 counts up every instruction cycle (and wraps around), A7 reflects the state of bit 7 of the R register (which only changes if software changes it) and A8 to A14 reflect the state of the I (interrupt) register (again, this will only change if software changes it).

You can tell when the address is a refresh cycle, as /RFSH goes low.

When the Z80 accesses memory, /MREQ goes low, then the address bus should have a valid memory address on A0 to A15.
When the Z80 accesses I/O (that is not memory mapped), /IORQ goes low, then the address bus should have a valid I/O address on A0 to A7 or on A0 to A15 (depends on the I/O instructon used).

Mark

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 10 Apr 2019 12:59

What is the Z80 /RESET line and the clock (pin 6) doing?

Are the ROM chips in the correct sockets?

Are the ROM pins making good contact with the legs of the chips.
Do the same checks with the PAL.

Mark

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 10 Apr 2019 13:10

The /RESET line stays high.

The ROM were originally never removed. I've removed and reseated them, similarly the PAL (white dot).

ROM A is in the middle socket, ROM B in the upper socket and ROM C in the lower socket. This is a 4000-05 board.

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 10 Apr 2019 13:11

1024MAK wrote:
10 Apr 2019 12:58
Every instruction cycle the Z80 will generate a DRAM refresh address on A0 to A15, the state of A0 to A6 counts up every instruction cycle (and wraps around), A7 reflects the state of bit 7 of the R register (which only changes if software changes it) and A8 to A14 reflect the state of the I (interrupt) register (again, this will only change if software changes it).

You can tell when the address is a refresh cycle, as /RFSH goes low.

When the Z80 accesses memory, /MREQ goes low, then the address bus should have a valid memory address on A0 to A15.
When the Z80 accesses I/O (that is not memory mapped), /IORQ goes low, then the address bus should have a valid I/O address on A0 to A7 or on A0 to A15 (depends on the I/O instructon used).

Mark
I'm going to have to have a way to set up a logic analyser aren't I? The oscilloscope is just not going to cut it.

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 10 Apr 2019 13:50

The /RESET line should go low then high again if the two keyboard reset keys are both pressed together and then released. Or if the keyboard is disconnected, short the relevant pin on the keyboard connector (on the main board) to 0V briefly.

If you have a breadboard and some suitable 74xxx series logic gates, you can use these to combine some of the Z80 control signals and trigger a ‘scope.

Obviously a four channel ‘scope or a logic analyser would be better.

Mark

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 10 Apr 2019 14:59

I can cause a reset using the keyboard. The tone goes off and then comes back again.

I have a cheap USB logic analyser but nothing I can use to plug it into. I'm thinking that the next step may be to buy an edge connector and wire-wrap board/pins and make a break-out board for all the bus signals so I can easily plug the logic analyser into it. This will probably have to wait until after Easter now as I'm away for two weeks after this weekend. Do you have the specification for the MTX edge connector? I can at least order parts from Farnell or RS which should arrive before the weekend.

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 10 Apr 2019 15:33

The edge-connector details are in the Memotech BASIC Tutor Reference and Operators Manual / Memotech Operator's Manual (on one of the pages that has the schematic diagrams) and also in the MTX 500/512 Service Manual and Technical Information manual (PDF page 23, paper page 20).

Visit Dave’s web site to download copies here ;-)

Dave has lots of useful information on his web site. So be careful, you will be there for a while... :lol:

Mark
Last edited by 1024MAK on 10 Apr 2019 15:43, edited 1 time in total.

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 10 Apr 2019 15:41

1024MAK wrote:
10 Apr 2019 15:33

Dave has lots of useful information on his web site. So be careful, you will be there for a while... :lol:

Mark
:D

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 10 Apr 2019 17:46

1024MAK wrote:
10 Apr 2019 15:33
The edge-connector details are in the Memotech BASIC Tutor Reference and Operators Manual / Memotech Operator's Manual (on one of the pages that has the schematic diagrams) and also in the MTX 500/512 Service Manual and Technical Information manual (PDF page 23, paper page 20).
Does that give all the contact pitch and board thickness details? (I can't have a look at the moment as I'm at work.)

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