Donor challenge: Your generous donation will be matched 2-to-1 right now. Your $5 becomes $15! Dear Internet Archive Supporter,. I ask only. Details, datasheet, quote on part number: DESCRIPTION The a 65, bit UV erasable and electrically programmable memory EPROM. It is organized. High speed performance. – ns access time available. • CMOS Technology for low power consumption. – 20 mA Active current. – µA Standby current.

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From time to time those of us with the passion to preserve ancient computers from the and 22764 will come across the need to replace a failed masked ROM or EPROM.

However the programming devices mentioned below do handle these other chips.

IC Datasheet: 2764 EPROM

Furthermore, the article doesn’t cover all of the theory and physics. Rather it provides enough practical tips and information for readers to attempt an EPROM replacement of failed compatible parts.

The ICs hold code, often an auxiliary program such as an application, utility or game which can be loaded immediately on startup. Mask ROMS have embedded code that can’t be changed.

Once reprogrammed or “burned”they can hold their data for many years. For your average vintage microcomputer, EPROMS can be recognised by their clear transparent windows, normally covered 27664 some kind of label. Usually these Eptom start with either a or a prefix but there may be others. After the prefix is a number such as 16, or 32, 64, etc.

There are 8 kilobits in a kilobyte. A can hold 4k and so on. These have a “C” between the prefix and size number e.

This is why the ICs have a clear window in them; to allow the UV light to penetrate and erase the data. Fig 1 shows my eraser.

These are inexpensive devices that can be found on eBay. Typically, my eraser will blank a chip in about 20 minutes. Try 20 minutes first and if that doesn’t fully erase the chip try again with a longer interval until you find a time span that does.


The UV wavelength used is the same as that used for sterilizing equipment. It is not the harmless UV found in disco lights. Don’t look directly at the bulb when the door is open. One is the READ voltage. This is applied to the Vcc pin and is typically 5 datadheet.

A Datasheet(PDF) – STMicroelectronics

The other voltage is the programming e;rom Vpp. Typical values are 25v, 21v or Here is an important piece of advice. The incorrect voltage can kill it. Not just the size but also the manufacturer. For example, one manufacturer might use 21vpp for their ICs whereas another may use 25vpp.

Make sure you know the specs you’re dealing with. There are fundamental differences between the “non-standard” TMS made by Fprom Instruments and ‘standard” s made by others. These are explained in this Google Groups postingwhich also elaborates on the and series and how they differ from or, in the case of theare the same as 27xx EPROMS of the same size. Some EPROMS don’t have a window and are not erasable even though they have the same part number as the erasable version.

Be sure to check when ordering and make sure you get the UV-erasable version unless you want an OTP of course. First rule is to know the kind of EPROMS you want or are likely to want to burn and get a programmer that can handle them.

There are a number of programmers on the market.

2764A – 2764A 64K EPROM Datasheet

Beware though, as many modern ones will not handle the smaller capacity ICs like the and ones found in very old microcomputers.

Willem programmers appear popular fig 2. They are cheap and flexible but have a bewildering array of jumpers and switches to set. I had a Willem but it died so my current programmer is a Top out of China fig 3.

It’s also inexpensive, is completely software driven and can supply all necessary programming voltages from a USB link. It seems to work well, although at the 276 of writing the manual for this Chinese device appears to have been written using an early version of Google Translator! Eprm almost completely unreadable. If you know the basics though the software is easy enough to follow.


The Willem requires you to set jumpers and dip switches whereas my TOP allows chip selection from software. In particular make sure the Vpp is correct and that the IC is oriented correctly and in the right position on the IC holder.

The software that comes with the programmer will manage the burning process. Modern programmers use menu-based interfaces that are easy to use. With the chip in place, the first step is to load the code into a buffer prior to burning it into the EPROM. The code needs to be a binary file. Figure 4 shows a screen shot of my programmer with a 4k System 80 ROM image loaded.

TOPS software showing a binary loaded into the buffer and ready to go! There will be an icon or menu selection for this. Usually a status datashret shows progress.

Daatasheet complete, most software allows you to verify that the image in the EPROM is now the same as the one in the buffer. It may even check automatically. If things are OK, you’re done. Often the pin-outs of the mask ROM will be different to the EPROM and some kind of “adapter” needs to be made to switch the pins around or the board circuitry altered. Here is one of my efforts regarding the latter. What you need to do will differ depending the machine.

EPROMS , , , , , , ,

Otherwise there is great risk of zapping the ROM epom voltage is applied to the wrong pin! Getting to grips with EPROM programming, and having the gear to do it, is worthwhile if you want to maintain a vintage computer collection. Not all ROM images are available on the Web and it’s a good idea to extract the code for posterity and possible future use!

This article is just a basic primer. Datasjeet couple of VCF folks helped me refine this article.