Not for just "protecting the poor EEPROMs", but modern ones does not need extra voltage (like +12V) at all, and no need for complex write cycles either, in fact they are easier to write than flash, and a single byte can be modified as well (flash needs block level operation), unlike on flash. Modern EEPROM devices in fact still requires stuffs Zozo wrote about, but it's done/generated automatically by the IC itself. The write is the same as with a RAM, no difference at all (it's quite different with flash)! Ok, maybe one: you can't write EEPROMs rapidly, you "should" wait after a write operation, because it's still much slower than a read. But anyway, I guess EEPROMs are still more expensive, but they're worth for hobby purposeses anyway if you need low capacity. I am quite sure that for eg 512K or so, you should forget EEPROMs anyway

Lon ago Zozo installed in my EXDOS card a 128k EPROM, replacing the board ROM socket of 28 pins with a bigger  of 32 pins.

Yes, your card originally has 3x32K (28 pins), it can be modified up to 3x512K (32 pins), recommended 512K Flash ROM, and 2x512K SRAM. Then you got a total 1.5M RAM expansion, and 512K for ROM programs.
Details will come, but I need to work now

What kind of EXOS PCB was?. The only one I've found with 3 sockets is the EPPlus one.
Can I modify its board to work with 32 pin EPROMs?

About the pinouts, lets start where the story begin:
2708 (1Kbytes) and olders (2704, 2702) are the great-grandfather of all ROM chips what we using.

These use three power lines (+5V,-5V,+12V), then can not be used in modern computer systems such as the Enterprise.
But look the pinout: Data and Address lines positions will be standard for the many next generation!

The next generation of the 24 pins EPROMs the 2716, 2732 (2K,4K):


These the first single power line EPROMs. Another change now the chips have separate Chip Enable and Output Enable signals, this is will be standard for the future generations.
In Enterprise systems only printers using this small capacity chips. (At ZX Spectrum and Commodore systems many hw using this type.)

Because all pins used at 2732 the next step need a longer package then 2764 (8K) are the first 28 pins EPROM.

In Enterprise only used at some early EP64 machines where the IS-BASIC stored in 2x8K EPROMs.

The next series the 27128 (16K) and 27256 (32K) the usually used when the Enterprise born.


-motherboard have a 32K socket
-one socket cartridges have a 32K socket
-two sockets cartridges jumperable 8K/16K. Some early EP64 machines have the 2x8K configuration, later machines and all English EP128 machines got a one socket cartridges with 16K ROM chip. German EP128 machines have a mixed cartridges: two sockets type with 2x16K ROM, or one socket type with 32K ROM.
-EXDOS card have a jumperable 16K/32K socket, the standalone card with 1.0 version are 16K, built in card in the German floppy units have a version 1.3 ROM which are 32K
-third party EXDOS cards have a 32K socket, MICROTEAM type have a 3x32K socket
-the Plus cartridge from the 'a' Studio have a 2x16K and 1x32K sockets
-exist a third party cartridge with 3x32K sockets, where 2x32K are switchable

At 16K sockets the Pin 27 connected to VCC (Pin 28). Modify to 32K need to cut this and connect Pin 27 to A14 address line. This is very common modification for the two sockets cartridges. Also need to modify the address decoding on cartridge, remove the jumper and connect the A15 to the decoder IC (74ALS138) input.

The next big evolution step when the 27512 (64K) chips are arrived at the end of '80 years.

We started using at the one socket type cartridges, the modification are easy: remove VCC from Pin 1 and connect to A15.
Because also 64K address space decoded for the onboard EXOS ROM I do same trick in the motherboard and started the EXOS 2.2, 2.3 evolution.
The original EXOS 2.0,2.1 only search ROM at the 10h,20h,30h,etc segments, but my updated versions search all segments. Then possible using 64K EPROMs on expansion cards, with more programs in one EPROM. The trick is same: connect Pin 1 to A15. MICROTEAM type cards now can be used with 3x64K ROMs.

There is a pinout summary from 2716 to 27512:


At the '90 years arrived the 128K, 256K EPROMs, these need a more address lines, then got a 32 pins package.


At the cartridge or motherboard only 64K address space decoded, but on EXDOS cards 256K decoded for sockets. Then possible to use this bigger EPROMs. Firstly need modified up to 64K socket.
Adding 27C010 are easy, because the Pin 30 not used which are the Pin 28=VCC in the socket, Connect the EPROM Pin 30 to Pin 32 then the IC have a power. And connect EPROM Pin 2 to A16.
I do this for example on Gflorez card.

27C020 need a A17 at Pin 30 then the previously VCC trick not working,  need a separate VCC wire for Pin 32. And bend up the Pin 30 for don't connect with the socket Pin 28, and connect to A17. And Pin 2 to A16 as the 27C010.

Modify MICROTEAM type EXDOS cards for 3x512K address decoding to the ROM sockets.
Schematic of the card.
IC positions.

Originaly 3x256K address space decoded for the ROM sockets: 10h-1Fh,20h-2Fh,30h-3Fh segments.
And 2x256k for the DRAM expansion, 40h-4Fh,50-5Fh segmens

For double the address space need to remove A21 from IC12 pin 10 then connect it to GND. Now the problem the DRAM address space also doubled, RAM segments are mirrored. For correct this remove GND from IC19 pin 2 then connect it to A21.
Then the ROM sockets decoded to: 10h-1Fh+90h-9Fh,20h-2Fh+A0-AFh,30H-3Fh+B0h-BFh. DRAMs still on 40h-4Fh,50-5Fh.
When installing 512K ICs to the ROM sockets connect A21 address line as A18 at the ICs.

One cut at the top of card:


Another at the bottom:


Wiring:

I understand the flash roms can be burned "in situ" but, whouldn`t it interfere with the operation of EP if EXOS or ExDOS roms are over-written while been accessed?

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I`ve reading data sheets about 29X040 ROMs, and some allow to be erased  and burned partially in banks. Which is the best candidate for you? Can you name the exact reference of the necessary chips?

 Thanks zozo, great write up!