I bought 10-12 eproms/eeproms more than a year ago on eBay, which until recently, I hadn't had a reason to play with. I am in the process of repairing a couple of 8-bit retro computers (Oric-1's) and seeing as the first thing any self-respecting Oric owner should do is upgrade the Oric's ROM Basic from v1.0 to v1.1 (making it an Atmos essentially), I went a searching for my Eproms.
I had bought equal quantities of low-power CMOS 27c64 & 27c128 eproms, all of which sported the 'ST' label - meaningless, I know, coming from China. Anyway, I flashed one, stuck it into the Oric, floppy-disk micro-controller attached, and, nothing - it refused to boot. Detaching the floppy, everything was roses. So I pulled a 'working' eprom from my Oric Atmos, stuck it in the Oric-1 with floppy attached, and this time the Oric-1 would load the DOS in without issue. Clearly, something was 'different' about these Chinese eproms.
Seeing as it's much easier to check power usage with a battery-powered appliance, I elected to do so using old faithful, the RCT Controller, mentioned in previous posts. What I discovered was that despite being labeled as being CMOS devices, these eproms are nothing of the sort.
Below is a series of pics showing the combined current being drawn by the RTC controller from a 9V rechargeable battery. I start off with 'known-quantity' eproms, those I have had for decades and most likely bought from Cricklewood Electronics, London. The first one is a non-CMOS 27128, this draws about 60mA. Next in line is a genuine CMOS 27c128. This draws less than half the current of the previous one, about 29mA. Next come the impostors, first a Chinese 27c64, which pulls around 55mA, followed by a 27c128, drawing nearly 60mA. For variety, I've included a couple of EEPROMS, an Amtel 27c64 and a Xicor 28c256. These almost certainly Chinese knock-offs, at least seem CMOS in nature, both drawing around 30mA.
What pisses me off is that I've gone from having what I considered a 'plethora' of eproms, to having about 3 that perform to specs. It would be 4 but for the fact that I UV-wiped my one genuine 27c64 during the last (and final) edit of the RCT s/w, and it never recovered - not all bits would reset. Which means I will also have to go looking for more, this time from a reliable source.
Edit:
The plot thickens. Things are less straightforward than I thought. First, the original 16KB Oric ROM has pin 27 labeled O/E (output/enable), whereas it's the PGM (programming) pin on 27128 eproms. Since the Oric's Microdisk controller's ROMDIS (Rom-disable) is directly tied to this, it should explain perfectly why an Oric-1, hooked up to a Microdisk floppy-drive system, will not boot. The complication arises with my Atmos, which I modified (decades ago!) to work with eproms, so that I could upgrade from the original buggy & slow v1.0 Basic, as I explained above. So presently, I am still at a loss as to why these Chinese eproms will not work with my Atmos/floppy-drive setup.
Edit1:
Stranger & stranger. I've just realised that my unmodified Oric-1, with the Microdisk floppy-drive attached, and fitted with my 'magic' eprom (TMS27c12820JL), will boot & run perfectly! This makes no sense whatsoever!!! This eprom was bought & flashed in the 90's. Since the modified Atmos, and the unmodified Oric-1 operate perfectly with it, and the Microdisk attached, it stands to reason that something nefarious is afoot. Seriously, should be impossible.
