128K ROM?
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128K ROM?
Might be a silly question, but would a 128K ROM work in a 48K Spectrum? Not expecting to use 128K games, but would the 48K option work?
I’ve got one on the way for a future Harlequin 128K build and would like to test it at least functions….
Could I drop it into a Retroleum plug in card instead of the supplied diag ROM?
I’ve got one on the way for a future Harlequin 128K build and would like to test it at least functions….
Could I drop it into a Retroleum plug in card instead of the supplied diag ROM?
- 1024MAK
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Re: 128K ROM?
Yes and no.
The ROM area in all ZX Spectrum computers is 16K bytes.
The ROM chip in a 128K (toastrack, +2 grey) is 32K bytes. The top address line is used to bank switch between each half. One half is the 128K menu, 128K full screen editor and the 128K basic. The other half is the “48K” BASIC.
So if you modify the 48K Spectrum (*), and apply the appropriate logic level to address pin A14, yes it should work.
(*) the ROM used in a 16K/48K/+ ZX Spectrum has two /CS (/CE) control inputs and a /OE control input (total of three), but 128K ROM and 27256 (or 27C256) EPROM chips only have a total of two. Plus the different ROM manufacturers used slightly different pin-outs for the control signals. Hence a small amount of rewiring is needed.
Mark
The ROM area in all ZX Spectrum computers is 16K bytes.
The ROM chip in a 128K (toastrack, +2 grey) is 32K bytes. The top address line is used to bank switch between each half. One half is the 128K menu, 128K full screen editor and the 128K basic. The other half is the “48K” BASIC.
So if you modify the 48K Spectrum (*), and apply the appropriate logic level to address pin A14, yes it should work.
(*) the ROM used in a 16K/48K/+ ZX Spectrum has two /CS (/CE) control inputs and a /OE control input (total of three), but 128K ROM and 27256 (or 27C256) EPROM chips only have a total of two. Plus the different ROM manufacturers used slightly different pin-outs for the control signals. Hence a small amount of rewiring is needed.
Mark
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Re: 128K ROM?
Thanks Mark,
I think I understand, but forgive me, if the Retroleum diag board uses an EPROM, would this not have the required pinouts for a 128K+ ROM?
I think I understand, but forgive me, if the Retroleum diag board uses an EPROM, would this not have the required pinouts for a 128K+ ROM?
- 1024MAK
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Re: 128K ROM?
Sorry, must have been tired when I read your previous post, hence I may have answered a question you didn’t ask
Yes, the Retroleum ROM board can take the ROM chip from a 128K machine (toastrack, +2, +2A, +2B, +3). These ROMs have the same pin-out as a 27256 or 27C256 EPROM. Which will work in a socket designed for a 27512 / 27C512 EPROM.
Mark
Yes, the Retroleum ROM board can take the ROM chip from a 128K machine (toastrack, +2, +2A, +2B, +3). These ROMs have the same pin-out as a 27256 or 27C256 EPROM. Which will work in a socket designed for a 27512 / 27C512 EPROM.
Mark
ZX81 Variations
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Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
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Re: 128K ROM?
Thanks Mark,
I'll give it a try when it arrives
I'll give it a try when it arrives
- 1024MAK
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Re: 128K ROM?
Although I should add that the DIP switches must be set correctly.
Mark
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
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- 1024MAK
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- Joined: Mon Sep 26, 2011 10:56 am
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Re: 128K ROM?
For a 27256/27C256/32K byte ROM
Pin 1 should be +5V (A15 on a 27512, VPP on a 27256)
Pin 27 (A14) should be +5V to select the top half (which is where the “48K” ROM is IIRC).
Mark
Pin 1 should be +5V (A15 on a 27512, VPP on a 27256)
Pin 27 (A14) should be +5V to select the top half (which is where the “48K” ROM is IIRC).
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
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Re: 128K ROM?
I'll check on the instructions to see how to set the switches to use the upper half
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- Posts: 190
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Re: 128K ROM?
ROM arrived today, put it in the Retroleum card, switches set for 2nd 16K set out of 4. Connected to my creaky series 2 with the internal ROM removed (to make sure it was the external ROM being used) and it booted fine.
Very happy for just over a tenner. Hopefully Santa can bring me a Harlequin 128K kit
Very happy for just over a tenner. Hopefully Santa can bring me a Harlequin 128K kit