Who wants a MAGROM board?

Everything about Martin Allcorn's Games ROM
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Dave
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Who wants a MAGROM board?

Post by Dave » 28 Nov 2013 00:28

http://primrosebank.net/computers/mtx/p ... magrom.htm

Martin has designed a nice external ROM board and I would like to get one.

If there is enough interest, it may be worth considering getting some professionally made, using the same source as Andy, the board would likely cost <£5.

Is there any interest in going that way?

Regards
Dave

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1024MAK
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Re: Who wants a MAGROM board?

Post by 1024MAK » 28 Nov 2013 02:02

Oh yes! :D

I would be interested in two boards.

Mark

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Re: Who wants a MAGROM board?

Post by 1024MAK » 29 Nov 2013 03:28

About the ROM board. Having had a quick look, I think even if we stay with "TTL" logic chips and one ROM, the chip count can be reduced by at least one, may be more ;).

Mark

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Dave
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Re: Who wants a MAGROM board?

Post by Dave » 29 Nov 2013 09:51

Hi Mark,

Maybe you could describe what you're thinking and Martin could take a look at it?

Regards
Dave

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Re: Who wants a MAGROM board?

Post by 1024MAK » 29 Nov 2013 11:21

Of course ;)

Details soon...

Mark

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Re: Who wants a MAGROM board?

Post by 1024MAK » 30 Nov 2013 00:57

Okay, this draft amendment schematic reduces the chip count by two :D .
U1, U6, U8 stay the same type (but U1 and U8 have some wiring changes) as per Martin's schematic.
U2 has now been changed to a 74LS00 NAND chip.
U3 has been deleted.
U4 has now been changed to a 74LS30 NAND chip.
U5 has now been changed to a 74LS30 NAND chip, but if preferred can stay as a 74LS138 (just use Martin's schematic).
U7 has now been changed to a 74HCT374 or a 74HCT574 (these both provide the same function, but have different pin-outs).
U9 has been deleted.
RN1, a 4.7k resister network has been added (7 pin 6 resistor network or larger). Normal resistors can be used instead.
MAGROM_mods_draft(50).png
MAGROM_mods_draft(50).png (226.3 KiB) Viewed 3161 times
As the latch now used (U7) does not have a clear/reset input, the "firmware" running in "ROM7" (address block 02000h to 03FFFh)
of the EPROM/FLASH (U8) should make sure that the correct value is latched before accessing the EPROM/FLASH (U8) as "RAM".
There is no problem accessing the correct address range when the EPROM/FLASH (U8) is accessed as "ROM7" (address block
02000h to 03FFFh), as when address line A14 is low, U2A inverts this signal and it is applied to the three state /OE input of the latch
(U7) which disconnects the outputs and puts them into a high impedance condition. This allows resistor network (RN1) to pull down
the address lines A14 to A19 of the EPROM/FLASH (U8) selecting the lower memory block.

Hopefully my brain was fully operational when I doodled this and I have not made any logic (or other!) errors. Feel free to shout out
if I have dropped a boo-boo :mrgreen:

Mark

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Re: Who wants a MAGROM board?

Post by Dave » 30 Nov 2013 01:07

Hi Mark,

Thanks for that, I'll pass the info on to Martin,

Regards
Dave

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Re: Who wants a MAGROM board?

Post by Martin A » 30 Nov 2013 12:46

@ Mark,

Replacing the 74xx20 and 74xx138 with a 74xx30 and one of the unused gated on 74xx32 seems so obvious now you've mentioned it.

The trick with the 74HCT374 and the resistor pack, just shows the huge gaps in my knowledge, I just mirrored the paging system Memotech used.

One thing I was curious about, but couldn't be sure I understood, and as my test bed MTX has no PAL I chose not to try and use it, is the *GROM signal on the edge connector.

Is it a active low from the MTX for ROM 7, or is it an active low from the expansion that says a game cartridge is fitted, and forces the paged ROM access so select page 7? (page 226 of the new manual) that combination of resistors and diodes is doing something...

If it IS a active low for ROM 7 than U5 can be eliminated too. If its the other then it would break any game that relies on ROM routines in that area, and possibly the controller software too as that uses a couple of RST's in initialising.

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Re: Who wants a MAGROM board?

Post by Dave » 30 Nov 2013 14:34

Martin, Mark,

I wrote this page when I was trying to understand the MTX ROM select logic...

http://www.primrosebank.net/computers/m ... m_mtx4.htm

E&OE excepted of course :-)

Hopefully, it is on the right lines - as ever, feedback welcome

Regards
Dave

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Re: Who wants a MAGROM board?

Post by 1024MAK » 30 Nov 2013 19:26

Not having looked at a schematic of a game cartridge (let alone getting my hands on a real game cartridge), I cannot be sure if they use /GROM.

However, looking at schematic for the MTX main board, these are my thoughts...

First of all, keep in mind that R0, R1 and R2 originate from a 74LS273 latch (5A) and that they should not be driven by any external circuitry.

Diagram from Dave's site:
Image

The way that resistor R13 (4.7k) and the three 1N4148 diodes works is as a wired AND gate.

If R0, R1 and R2 are all logic high (logic 1), none of the three 1N4148 diodes will conduct and resistor R13 (4.7k) will pull input pin 9 of inverter 6B (74LS04) and input pin 12 of NAND gate 5B (74LS00) high.

If either R0, or R1 or R2 go low, one (or more) of the three 1N4148 diodes will conduct, which will pull pin 9 of inverter 6B (74LS04) and input pin 12 of NAND gate 5B (74LS00) low.

So the /GROM output can only go active if R0, R1 and R2 are high and the output O2 from the PAL (6A) is low.

This looks like it was added because of a limitation in the PAL (6A) (not enough pins) or because maybe the /GROM signal was added as an after thought.

Code: Select all

[¯¯¯¯¯¯¯¯¯address¯¯¯¯¯¯¯¯]
A15 A14 A13  (hex)  (dec) 
 0   0   0    0000      0  Monitor A (OS) ROM
 0   0   1    2000   8192  1 of 8 ROMs selected by R0, R1 & R2
 0   1   X    4000  16384  16k RAM (not present on MTX500)
 1   0   X    8000  32768  16k RAM (selected by P0, P1, P2 & P3)
 1   1   X    C000  49152  16k RAM (common)
               to
 1   1   1    FFFF  65535  End of Z80 CPU address space

[ROM selection]
 R2 R1 R0 (dec)
  0  0  0   0   SYS B - BASIC
  0  0  1   1   SYS C - Assem
  0  1  0   2   ROM 2
  0  1  1   3   ROM 3
  1  0  0   4   Disk - CP/M boot ROM
  1  0  1   5   Disk - SDX ROM
  1  1  0   6   ROM 6
  1  1  1   7   Cartridge
It would need testing to confirm that /GROM does what we expect, as it does depend on the logic equations in the PAL being correct.

Mark :mrgreen:

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