EPROM and EEPROM Compatibility

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roobarb!
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EPROM and EEPROM Compatibility

Post by roobarb! »

I have a bunch of EPROMs and EEPROMs here, all system pulls and in varying degrees of dishevelment.

Some of them, such as the 2764 (8KB) and 27128 (16KB) ones are obviously straightforward. The 27256 (32KB) ones I just offset the single 16KB ROM image to 4000 for the Model B and they work just fine, or I understand that I can pop an extra ROM in from 00000 to use with the Master.

However, I've also got 27512 (64KB) and some AT28C256 EEPROMs here too, which I've had mixed results with.

Can the 27512 EPROMs be used in the same way as 27256 ones with an offset and where should that offset be? I've tried 4000 and C000 which often leads to display scrambling and weirdness on resets, but as these are system pulls they could be damaged, so not knowing the expected behaviour causes me issues!

As for the AT28C256 EEPROMs, I often have the same trouble with screen corruption on resets and general misbehaviour. I remember reading something about programming these from 0, but also read that they can be twitchy on resets, so any advice on how to handle these as well?

Thanks everyone!
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KenLowe
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Re: EPROM and EEPROM Compatibility

Post by KenLowe »

The 27256 ROM gives access to 2 x 16k ROM banks. These are switched in and out using A14 on pin 1. Based on the beeb circuit diagram, pin 1 is tied to +5v, so will permanently switch in the upper 16k bank (&4000 in the ROM). You can make a minor wiring mod to the beeb so that pin 1 is switched by the ROM select circuitry, giving access to both upper and lower banks.

The 27512 ROM gives access to 4 x 16k ROM banks. These are switched in out out using A14 and A15 on pins 27 & 1 respectively. Again, based on the beeb circuit diagram, pin 27 is also tied to +5v, so if you try using it in a standard beeb socket, the 16k bank should map to &C000 in the ROM as you've indicated.

If either of pins 1 or 27 on the beeb are floating, then you will get issues as other unprogrammed banks may randomly switch in and out.

EEPROMs are similar to ROMs in terms of bank switching, but if you've made a wiring mod so the EEPROM can be programmed in situ, then a write protect switch MUST be used during normal operation otherwise strange behaviour will occur. This is discussed in detail on the 'Holy Grail' thread in this forum.
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

Thanks for this, I'll give another couple of 27512s a go at C000 to see if they play nicely. The Beeb is unmodified. Well, sockets 14 and 15 are unmodified; I have an IFEL RAM/ROM board in there with its top two banks disabled while I test these things.

I suppose one easy fix with the EEPROMs, or indeed any of these when I only want one ROM, is just to concatenate the image two or four times. But surely that's cheating! :wink:
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Re: EPROM and EEPROM Compatibility

Post by JasonStonier »

I've got most of my ROM sockets populated with BooBip EPROM adapters which does the wiring mod for you. I've got a 27C512 EPROM with 4x ROMs on it and it works very well. For about £12 each, in my view they're indispensable.
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Re: EPROM and EEPROM Compatibility

Post by cmorley »

JasonStonier wrote:
Wed Nov 18, 2020 3:03 pm
I've got most of my ROM sockets populated with BooBip EPROM adapters... ...in my view they're indispensable.
8)

Cheque is in the post...
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Re: EPROM and EEPROM Compatibility

Post by JasonStonier »

:lol:

I genuinely love your products, they're such an elegant solution and they -just work-. Couldn't be without them!
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Re: EPROM and EEPROM Compatibility

Post by Bobbi »

Seconded - loving my BooBip things (thinking to order more to fill the remaining sockets!)
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

JasonStonier wrote:
Wed Nov 18, 2020 3:03 pm
BooBip EPROM adapters
What is this magic of which you speak! Heading to the search box...
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Re: EPROM and EEPROM Compatibility

Post by Bobbi »

I have one of the BooBip 64KB EEPROM thingies and one of the 32KB Sideways RAM + 32KB ROM socket ones.

Super handy for developing or fooling around with ROM images. You can flash the EEPROM in situ in the Beeb using a BASIC program. Very cool.

The sideways RAM is also cool, because I can keep PAGE at 0E0 with MMFS/MMFS2.
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

Bobbi wrote:
Wed Nov 18, 2020 11:37 pm
I have one of the BooBip 64KB EEPROM thingies and one of the 32KB Sideways RAM + 32KB ROM socket ones.

Super handy for developing or fooling around with ROM images. You can flash the EEPROM in situ in the Beeb using a BASIC program. Very cool.

The sideways RAM is also cool, because I can keep PAGE at 0E0 with MMFS/MMFS2.
Just having a read of the BooBip website, these look very nice indeed!

On my tinker Beeb I use one of the battery-backed IFEL RAM/ROM boards which I love, but I've been looking for something that would be compatible with my USA export Beeb. I'd considered making a carrier board for the RAM/ROM but this may be a better option, though without the cool SWRAM battery backup. :)

IMG_7891.jpeg

The required signals will be available on here somewhere!

@cmorley, do you think they might work?
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Re: EPROM and EEPROM Compatibility

Post by Bobbi »

Wow the wiring for the DIN sockets on the USA Beeb is pretty crazy!
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Re: EPROM and EEPROM Compatibility

Post by sweh »

roobarb! wrote:
Thu Nov 19, 2020 1:39 am
On my tinker Beeb I use one of the battery-backed IFEL RAM/ROM boards which I love, but I've been looking for something that would be compatible with my USA export Beeb.
Doesn't the IFEL board fit? The ROM slots _look_ equivalent to the standard Beeb, just located in a different place. Or is there a subtle difference in spacing?

A single slot board like the BooBip board will work, but you might need to hunt down the equivalent pins for addressing/write enable. Shouldn't be too hard.
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

sweh wrote:
Thu Nov 19, 2020 2:10 am
Doesn't the IFEL board fit? The ROM slots _look_ equivalent to the standard Beeb, just located in a different place. Or is there a subtle difference in spacing?
Sadly not, it carries the 6502, so relies on the relative spacing of the classic Beeb ROM sockets and the CPU socket.

IMG_0532.jpeg

Does a schematic of the export Beeb board exist anywhere? IIRC it isn't well documented. :)
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Re: EPROM and EEPROM Compatibility

Post by sweh »

Oh, that's a totally different IFEL board to the one I have! https://sweh.spuddy.org/Beeb/Ram_Rom/
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

Not seen that one before! I do really like Steve / IFEL's products, there's an attention to detail I appreciate and his documentation is clearly written for retro nerds like ourselves, but even those of us who've not touched a Beeb in decades (or ever). Look, even the CR1225 was put in facing the right way around! :)

Back to the EEPROM chat, here's a quick video of what happens with an AT28C256 attached to the system. Corruption ahoy!

https://youtu.be/ySrPt3prjrw

Wish I could embed these videos, does anyone know how?

This is using a concatenated ROM file, two copies of the same ROM written across the whole IC. The funny thing is that it seems to settle down after a CTRL-BREAK or two, then randomly get upset again.
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Re: EPROM and EEPROM Compatibility

Post by MartinB »

roobarb! wrote:Back to the EEPROM chat, here's a quick video of what happens with an AT28C256 attached to the system. Corruption ahoy!
You need to fit a write-protect switch to Pin 27 of the 28C256 such that this pin is tied high (Vcc) for normal use after writing. Here’s a snip from the ‘Holy Grail’ thread which summarises the issue. I was answering a question about the Atom but the effect applies to all the machine types on our radar....

I wrote:Regarding a write protect switch, although the EEPROM can be software-locked and if it is written-to it won't therefore corrupt, it will go offline for about 10ms after the write attempt. This means that on a Beeb or an Elk without a specific hardware EEPROM write-protect facility, at switch on or after a reset the EEPROM goes briefly offline and because of the behaviour of the Acorn OS where all sideways roms are interrogated at start-up, a system hang typically follows. Now, if the Atom doesn't behave like this and doesn't interrogate the ram/rom at start-up, then there shouldn't be a problem but if it does, you'd also likely need to tie Pin 27 high when using an E2 prom in place of a 27256 eprom.
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Re: EPROM and EEPROM Compatibility

Post by 1024MAK »

One important thing to note, is that the schematic diagrams for the model B that show pin 1 of the sideways ROM sockets being connected to +5V/Vcc are WRONG. In actual fact on a real machine, pin 1 of these sockets is not connected to anything. Hence if a memory chip (no matter which type) is plugged in that uses pin 1 for an input signal, this (electrically) floating input pin will tend to pick up random voltages from other signals in the machine. This will then cause strange symptoms as the processor will at times not be reading the correct data from the chip.

The machine was designed for 2764 (8KB) and 27128 (16KB) EPROMs and ROMs (there are some links / jumper shunts for the correct configuration). None of these chips use pin 1 of the socket in normal use.

27256 and 27C256 can be used. They have one more address input, this is on pin 27. Pin 27 is tied to +5V/Vcc. Hence why an offset is needed for the processor to see the data. The processor can’t see the bottom half because the address input on pin 27 always sees a logic high.

27512 and 27C512 can be used, but pin 1 has to be connected to a valid logic level as on these, this is another address input. If it is connected to +5V/Vcc, then the processor will only be able to access the top 16k byte area. The first 48k byte area of the chip will not be accessible.

And as Martin has said, AT28C256 EEPROMs can be used.

You may find these links useful:
See EEPROMs 16k / 32k,
simple use of AT28C256
and EPROMS/EEPROMS
The mighty ‘EEPROM - the Holy Grail of Sideways Rom’ topic is here.

One note: In the linked topics above, there may be links to RetroClinic’s suggested modifications for DIY sideways RAM. This includes a modification to use 27512 or 27C512 EPROMs. Keep in mind that the way he shows the wiring results in the order of the 16k byte images in the EPROM being different than you may expect.

When you come back up for air and have more questions, feel free to ask away...

Mark
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Re: EPROM and EEPROM Compatibility

Post by JasonStonier »

roobarb! wrote:
Wed Nov 18, 2020 11:30 pm
JasonStonier wrote:
Wed Nov 18, 2020 3:03 pm
BooBip EPROM adapters
What is this magic of which you speak! Heading to the search box...
I wouldn't think twice about it - I've got two of the standard EPROM adapters, and 1 adapter with Sideways RAM. They occupy the same footprint as the original socket so you can fit them side-by-side. I bought mine from cmorley on eBay but I imagine he sells on here as well. Those things are the best price/value upgrade you can make to a beeb in my opinion.
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Re: EPROM and EEPROM Compatibility

Post by 1024MAK »

The is a useful diagram showing and comparing the pin out of 24 and 28 pin EPROMs on this page and shown below:

Image

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Re: EPROM and EEPROM Compatibility

Post by 1024MAK »

roobarb! wrote:
Thu Nov 19, 2020 2:42 am
Wish I could embed these videos, does anyone know how?
Ahh, it’s kinda covered here. But because not all browsers apparently work with embedding YouTube videos‬, always provide a link as well.

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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

Thanks everyone, so if I'm reading this correctly I could tie pins 1 and 27 together to lift pin 1 permanently high. That would limit the socket to the upper 16KB and prevent the floating pin weirdness.

In theory this wouldn't impact the use of @cmorley's adapters as they can't be making use of pin 1 if it's just floating... riiiight...? :)
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Re: EPROM and EEPROM Compatibility

Post by cmorley »

Pin 1 and 27 have no connection on my adapters to the motherboard. They connect to the clips in the piggy back socket - as per Mark/Retroclinic's guide but without requiring soldering or removal of the motherboard from the case.
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Re: EPROM and EEPROM Compatibility

Post by JasonStonier »

roobarb! wrote:
Thu Nov 19, 2020 11:43 am
In theory this wouldn't impact the use of @cmorley's adapters...
You could have bought one in the time it's taken to work all this out :wink:

Just do it - it'll change your life.

(that cheque still in the post, @cmorley..?)
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

JasonStonier wrote:
Thu Nov 19, 2020 3:07 pm
You could have bought one in the time it's taken to work all this out :wink:
But I need to KNOW!

Me knowing stuff and it being relevant is in itself irrelevant. :lol:
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Re: EPROM and EEPROM Compatibility

Post by JasonStonier »

Oh, I hear you. Knowledge is beautiful and relevant in and of itself.

My problem is, as I advance into my mid-40s, every new bit of knowledge pushes something else out and since my brain operates on a FIFO buffer, it's currently 1st year university stuff that is being sacrificed for esoteric facts about manatees and Weird Al Yankovic*.
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Re: EPROM and EEPROM Compatibility

Post by JasonStonier »

*manatees deliberately eat plants that make them fart, and since they have exquisite control of their sphincters they use that gas to control their buoyancy.

In Amish Paradise, Weird Al recorded the final scene forwards while walking and singing backwards, just so that he could have a horse walking backwards past himself walking forwards when he reversed the film.

You're welcome.
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Re: EPROM and EEPROM Compatibility

Post by 1024MAK »

JasonStonier wrote:
Thu Nov 19, 2020 4:17 pm
Oh, I hear you. Knowledge is beautiful and relevant in and of itself.

My problem is, as I advance into my mid-40s, every new bit of knowledge pushes something else out and since my brain operates on a FIFO buffer, it's currently 1st year university stuff that is being sacrificed for esoteric facts about manatees and Weird Al Yankovic*.
Blooming spring chicken! Wait until you have another ten years on the clock... :lol:

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Re: EPROM and EEPROM Compatibility

Post by JasonStonier »

1024MAK wrote:
Thu Nov 19, 2020 4:29 pm
Blooming spring chicken! Wait until you have another ten years on the clock... :lol:
If I keep buying stuff for the beeb at my current rate, I won't live that long. Mrs. S has assured me of this.
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

JasonStonier wrote:
Thu Nov 19, 2020 5:03 pm
If I keep buying stuff for the beeb at my current rate, I won't live that long. Mrs. S has assured me of this.
Haha, well, make sure you pick up your cheque from @cmorley, @JasonStonier - a little order went in last night! ;)
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Re: EPROM and EEPROM Compatibility

Post by roobarb! »

roobarb! wrote:
Thu Nov 19, 2020 11:43 am
Thanks everyone, so if I'm reading this correctly I could tie pins 1 and 27 together to lift pin 1 permanently high. That would limit the socket to the upper 16KB and prevent the floating pin weirdness.
For completeness, connecting pins 1 and 27 together (or 28 which is VCC anyway) has indeed resulted in the AT28C256 EEPROM behaving perfectly.

I flashed two versions of the same ROM to 'upper' and 'lower' banks. With nothing connected, on the offchance I got a decent initialisation the lower ROM image was in use. With pin 1 high I got the upper image, as I would with a 27256 EPROM.

Nice easy bodge with a couple of probe hooks for testing purposes. :)
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