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Eprom Adventure

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fatwizard

Veteran Member
Joined
Sep 28, 2012
Messages
525
Location
Indiana, USA
Hi all,

I have been away from the hobby for a while as life had other ideas about how I spent my time. However, I had occasion lately to jump back in with both feet. I managed to snag a real nice Priam 60 meg MFM drive off of eBay as a birthday present to myself. It tested perfect, so I went about stuffing it in my 5160. While I had the old girl open, I thought I would make a backup of the BIOS ROM in my Jameco JE1077 multi I/O card as I had seen suggested here on the forum.

This card features a parallel, game, and two serial ports, a clock and a HD floppy controller. I have become quite dependent on having a 1.44 drive in the XT. Well, I didn't know how to identify the EPROM type in the card, so I thought I would try warming the chip up and peeling the label back to see the chip type. I managed to be inattentive at the wrong moment and got the chip too hot.

Sure enough, when I reinstalled the BIOS Chip, it was no longer functional. The chip turned out to be a 27C64, and it was a PROM, not an EPROM. So I ordered a 27C64 from eBay and whipped out my programmer. I downloaded all of the HD floppy controller ROM images from modem7's amazing site, minuszerodegrees, then I proceeded to erase and reprogrammed the 24C64 with each one in turn checking the results each time.

Well, I got very, very, VERY lucky. The last ROM image, for a Sunix brand SUN-4300 8bit floppy controller, was a perfect match! It has the same display on startup showing the floppy drive configuration. The drive select jumpers work as they did before. It even improved on the old BIOS by allowing me to format 1.44 disks without specifying the disk type with software switches after the format command.

The moral of the story is, I can screw it up all by myself, but with the amazing resources found here I was able to extract my bacon from the fire I started.

Very satisfying indeed.
 
" The chip turned out to be a 27C64, and it was a PROM, not an EPROM." Do you mean it was a "no window" EPROM, otherwise known as an OTP device?
 
I have my terms wrong. The original chip was labeled TS27C64A, and is indeed a windowless device. I know it's the same chip on the inside with just the expense of the quartz window omitted.

One time programmable = OTP I take it? Since the "E" in EPROM stands for "Erasable", I would have thought a "no window EPROM" a contradiction in terms.

I appreciate all enlightenment. Too many years working all by my self where I was the only one who needed to understand what I was talking about. :)
 
I've certainly seen the windowless versions called "OTP EPROMs" (yes: "One time programmable"). One of the many joys of terminology that doesn't make sense. Fortunately, we have EEPROMs which don't need any stinkin' windows. Matter of fact, I haven't purchased an UVEPROM in quite some time--EEPROMs are simply easier to deal with.
 
A PROM is technically different from a OTP EPROM. A.PROM can in theory never be erased. An OTPEPROM is the same inside as a "normal" EPROM. It can't be erased with a lamp. But, it has the same life expectancy of an EPROM, and, both devices can actually be erased with X-rays.

Ever since learning this, I've wondered how many PROMs are really OTP EPROMs.
 
But, were they cheaper than conventional PROMs? They must have been. Or, were what we knew as PROMs always really OTP EPROMs? I don't recall if I ever learnt the technical details of a PROM. But then, I don't think I've seen a PROM in thirty years anyway.
 
That was very stupid to heat up that prom before making an backup from that prom.. I always backup things before doing other unsave things.. Maybe a lesson for the next time..
 
Well a qquick read at Wikipedia clears that up...

i do now recall learning about the fuses in PROMs. So, genuine PROMs are indeed a different technology than OTP EPROMs.

And, apparently it takes a little more than a simple light dose of X-radiation to erase an EPROM. All hppes of erasing a OTP EPROM at home are completely dashed. :p
 
Robin4 said:
That was very stupid to heat up that prom before making an backup from that prom.. I always backup things before doing other unsave things.. Maybe a lesson for the next time..
Click to expand...

Well, being stupid is nothing new around here, but I just didn't know how I could back up the chip if I didn't know the type of chip it was. My Willem programmer expects to be set for the device type, and I didn't think I could trust a backup made while specifying the wrong chip.

There very well may be a method of determining the type of chip without reading the numbers on the device, but I don't know how. In retrospect, I shouldn't have been so concerned with the paper label, and more concerned with the chip. I also could have pulled the ROM code from the computer's memory, I just didn't think of that.

It's always a lesson for next time.
 
Right at the first post we knew you learned a lesson. Even if we're not trying to read a chip number, your post helps us stay conscious of our actions. Thanks, fatwizard.
 
fatwizard said:
There very well may be a method of determining the type of chip without reading the numbers on the device, but I don't know how. In retrospect, I shouldn't have been so concerned with the paper label, and more concerned with the chip. I also could have pulled the ROM code from the computer's memory, I just didn't think of that.
Click to expand...

There should be--the "Silicon Signature" for EPROMs was started by SEEQ donkey's years ago with the 27128, IIRC.
 
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