bipolar fusable PROMs

This Web page created Feb 12 2016. I'm consolidating work I've done on reading and writing bipolar fusable PROMs. These were in common use in the 1970's and 80's as small, fast programmable PROMS. Some systems used them as "microcode" as part of a TTL-based controller. Over several years, I've stumbled into these. In Feb 2016 I've consolidated and cross-referenced what I've collected onto this Web page, and link to the other Web pages where I've done or discussed this sort of work.

Caution: fusable PROMS do not have standard programming methods! They can be read with ordinary TTL circuits at 5 volts; but each brand and model and version require different programming methods. Read my comments about programming fusable PROMS and obtain the data sheets for your specific brand and model of PROM you wish to program.

Jeff Shook reads Digital Systems microcode
PROMs for Futuredata floppy controller
Adapter to read 7641 bipolar PROMS
PROMs for PDP 8/A boot code
alternatives to 74S387
Zilog MCZ uses PROMs for addressing
data sheets
technical background on fuses PROMS

Somewhat related as small programmed devices, are PAL and GAL devices. Here's a Web page about their use in S-100 computers, and programmers for them.

To email me, see see my ordering Web page for my email addresses.

Jeff Shook reads Digital Systems microcode

See details on my Digital Systems page which discusses Jeff's reader for the DSI floppy controller.

[PROM programmer]

Jeff Shook said in July 2010: "I actually built the programmer for another project some time before I became interested in how the [Digital Systems] FDC-II controller worked [in 1980]. There isn't much documentation of my PROM programmer. Just a schematic diagram and two pictures of the top and the bottom. The photo of the underside of the bipolar PROM programmer shows it modified for reading only. The yellow wire near the 39 ohm resistor has been removed to ensure that no program power could be applied."

"I think the circuit design may have come from a magazine I found on the internet some time ago a very poor scan of a hand drawn schematic by Keith Wilson for a similar programmer. I have tried to enhance it and now it is mostly readable. I have attached the schematic for my programmer and the pictures. I will write up some operating instructions later. It would not be difficult to automate the operation using one of the USB to three 8 bit i/o port adapters I see on the internet." - Jeff Shook

Caution: Each fuse-link or bipolar PROM brand and model, require THEIR OWN DC AND PULSE VOLTAGES FOR PROGRAMMING. Consult specific data sheets for the brand and model or version of PROM you hope to program, for programming details. Jeff's programmer was not designed or built for every brand and model of bipolar PROM. - Herb

PROMs for Futuredata floppy controller

[micropolis PROM & adapter]

See details on my Futuredata Web page

Micropolis made a number of non-FDC-chip floppy controllers in the mid-1970's. It used 82S181 biploar PROMS, not supported on most PROM writers. So I wired-up an IC socket, to enable my PROM programmer to read (not program) that device. - Herb Johnson

82S181 PROMs

The 10019 board has three 82S181 PROMs, which are fuse-link PROMs. They are identified from the corner toward the Z80 CPU as 80007-03-1B; 80007-03-3B; 80007-03-5B. The PCB has an inked number, 9015102. Also in the photo is my DIP socket wired as a PROM reader adapter (see below).

The 82S181 PROM has the same pinout(almost) as some 2716 UV EPROMs. But the 82S181 is a 5-volt only device; some 2716's use -5V as well. The Thompson brand ET2716 is a 5-volt only device. But two of the four chip-select lines to the 82S181 are different on the ET2716. Here's a little text note on how to wire a DIP socket to adapt an 82S181 to be read like a ET2716. the resulting hex file is half 0FFh's, half programmed data. The photo shows the wired-up socket I used with a PROM reader; the text note describes wiring and use. the adapter is only for reading, not for writing.

The three ROMS were read as ET2716's; the resulting Intel Hex file was text edited to remove the excess 00's or duplicate data. (See the Futuredata Web page for details.)

Again, details of the adapter are on the text document linked above. Here's a wiring diagram of the 24-pin socket between the 82S181, and the ZIP socket of the PROM programmer used to read:

82S181	pin		ET2716
Vcc ------- 24 -------- Vcc
A8  ------- 23 -------- A8
A9  ------- 22 -------- A9
/CE1 ->GND  21          Vpp=5V
/CE2 ------ 20 -------- /OE
CE3 ------- 19 -------- A10
CE4 -->+5V  18          /CE

Adapter to read 7641 bipolar PROMS


Joakim Larsson in Dec 2015: "I used a modified version of your adapter design [for the Futuredata floppy controller] and could successfully read out two Harris 7641 bipolar 512x8 proms today. You can see some pictures on the Swedish electronics forum [at this Web link]. It will eventually become a contribution to the MESS [computer emulator] project. Thanks!" - Joakim

The link is to the Elektronik - Allmänt forum, subject line "Laga Esselte 100 (skoldator". Google translate or other Web services may provide a translation of that Swedish language Web page. Here's an extracted photo of the socket Joakim wired-up.

here's the wiring he used, to read a 7641 as a ET2716:

7641-5 pin      ET2716
Vcc ------- 24 -------- Vcc
A8 -------- 23 -------- A8
NC -------- 22          A9
/CE1 ->GND  21          Vpp=5v
/CE2 ------ 20 -------- /OE
CE3 ------- 19 -------- A10
STR -->Vcc  18          /CE
GND ------- 12 -------- GND 

- Herb, with quotes from Joakim Larsson

Again - these are notes about reading these PROMS, not writing or programming them. These PROMS are designed to be read under normal 5V TTL circuits. Programming these PROMS requires specific pulses and specific DC voltages - refer to the data sheets for your specific brand and model or version of PROM for programming requirements.

M8317 boot PROM for PDP 8/A

Details and progress may be on my PDP 8/A repairs Web pages.

The PDP 8/A M8317 board, includes a pair of fuse-link bipolar PROMS to hold start up or "boot" code, of up to 128 instructions. However, my board was from an OEM 8/A, and it may have booted a custom cassette drive, so the ROMS in there may be irrelevant.

Reading the DEC schematics to interpret the PROM model... 23-0008A is DEC's unprogrammed part number, 256 X 4 each, 16 pins. ROMs #1 E82 and #2 E87 (socketed) have these pinouts. Address lines 1-7 and 15, data lines 9-12, 13 and 14 some kind of active-low select. There are 1K pullup resistors on them, this suggests open-collector outputs. Addressed in pairs they provide 12 bits of instruction and 4 bits of extended addressing. Programmed DEC versions may be part number 23-087A2, 23-088A2.

There's also soldered-in PROMs at M8317 board location E26 (32 X 8, 23-084A1) and E27 (256 X 4, 23-086A2). My photo shows those 086* part numbers in yellow, and show these are MMI brand parts

Note: For the M8315 (CPU board), DEC's programmed PROMS are I believe used for instruction decoding. They are mostly 256 X 4 but one is 1024 X 4,.

Guessing: DM74S387 or DM54S387 has the same addressing pins, the same data pins, the same active-low selects. Unprogrammed the outputs are low. When the two selects are low, the open-collector outputs (74S387) go loose (pulled high by resistors), the tristate outputs (74S287) go high-impedance.

From the Operator's Handbook: "There are three types of bootstrap ROMs used on the KM8-A. E82 and E87 have different labels for the different bootstrap ROMs. Switches on modules that have E82 and E87 (Figure 3-16) labeled 87A2 and 88A2 should be set according to Table 3-11. If E82 and E87 are labeled 158A2 and 159A2, use Table 3-12 for switch settings. For CL8 systems using the RX8-E, E82 and E87 are labeled 156A2 and 157A2; use Table 3-13. (If Table 3-13 is used, Auto-Restart address must be selected, i.e., S2-2 is OFF, S2-3 is OFF, and S2-4 is OFF.) "

In Jan-Feb 2016, I spent some time decoding the PROMS for the 8/A. See my Web page on my 8/A bootstrap PROM work.

alternatives to 74S387

(continuing from the PDP 8/A PROM discussion above:)

From Mike's Arcade at I found the list below. Seems these PROMS have the same pinouts and are either tristate TS or open collector OC. Unprogrammed prices seem to be undr $10 each. Jameco, Electronic Surplus,,, eBay of course has some of these. Some sellers offer programming.

V1.6 Jan 13,1999 
                 SIG     AMD    MMI      NAT    TI      HAR     FAR
7113 256x4 OC == 82S126, 27S20, 6300,   74S387, 24SA10, 7610
7114 256x4 TS == 82S129, 27S21, 6301,   74S287, 24S10,  7611, 93427

Zilog MCZ uses PROMs for addressing

Details and progress may be on my Zilog MCZ repairs Web pages.

I have two Zilog Z80 MCZ 1/20 development systems. In Aug 2009 I did some review and cleanup of them. These were sold by Zilog in the late 1970's for Z80 development, based on their own bus architecture. The linked Web page has details of the MCZ systems. There's discussion about the use of Harris 7603 Bipolar tri-state 32x8bit PROMs as address decoders. Peter Hill tried to replace the 7603 with TTL logic, but had problems.

data sheets

Here are some PROM data sheets and PROM cross reference lists from various PROM manufacturers. This content can be very hard to find today. Please note, you need to know the specific pulse and DC voltage requirements, of your brand and model of PROM, in order to determine how to program them. Detailed manual programmers may be included on some data sheets. - Herb Johnson

Here's a Phillips 82S123 data sheet
a JPEG of a Signetics cross-reference
and a larger cross-reference
a Signetics 82S23 data sheet with programming info
here's a National Semiconductor 82S188/288 data sheet with programming and cross-reference
AMD 27LS08 data sheet and programming
DM74S387 or DM54S387 data sheet
Signetics 82S126 and 82S129 data and programmer
HM7602, HM7603 Harris data sheet
Radio-Electronics Feb 1981 has some kind of PROM programmer project.

technical background on fused PROMS

Reading these old PROMS is not technically difficult, as they can be read using simple TTL logic and 5V DC power - that's their normal operation. The problem is programming, which is neither normal nor standardized.

Keep in mind how these PROMs work: you are blowing out nichrome or other metal-based fuses that represent stored binary values. These PROMS need specific features of programming voltages and pulse voltages and durations to blast out that metal link, without causing additional damage. It's a controlled destructive process. Blowing those fuses requires energy - power over time - to do that. Too much power will spatter metal in the logic cell; too little and the fuse may be unblown. Many of them also require higher DC voltages during programming. READ THE DATASHEETS for the specific brand and model of PROM you wish to program.

Each brand and model of PROM, has specific requirements for 1) DC operating voltage when under programming (which may not be 5V but a higher voltage; 2) a DC voltage for the programming pulse; 3) a specific pulse width in time, sometimes even a specific rise time for the pulse; 4) some delay between programming pulses, to allow the PROM to cool; 5) some suggested minimum number of pulses per bit, to insure complete programming.

All that programming information may (or may not) be in whatever data sheets you can find, for the specific old-stock brand and model of PROMS you are able to purchase. Some data sheets even show manual circuits for programming - their brand and specific model of PROM. Don't assume "one size fits all", don't assume one programmer for one brand/model/version will program another's.

The information on this Web page is provided to inform and assist - but without any warranty whatsoever. I'm not responsible for any loss of data, equipment, or injury. NO guarantees this information is complete, accurate or correct. Use this information entirely and completely at your own risk.

I welcome any additional information and corrections. This is hard information to find and gather. Thanks and good luck with your use of vintage equipment.

- Herb Johnson

Copyright © 2016 Herb Johnson

Herb Johnson
New Jersey, USA
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