I'm planning to write iPXE to boot ROM of my 3c509b to enable HTTP booting on my old computer.

What is the type of the ROM chip and where would you obtain it? It's 28 pin connector, please see picture below.

Also, if you have done it yourself, I appreciate sharing experiences.

Etherlink III

  • Vogons mention it's "standard 28-pin", but what's the standard? :) vogonswiki.com/index.php/3Com_3c509
    – Zouppen
    Commented Aug 1, 2023 at 11:57
  • 5
    Standard means JEDEC standard pinout for DIP memory. Also, please do not put additional information into comments. better edit the question to include it right away.
    – Raffzahn
    Commented Aug 1, 2023 at 13:14

2 Answers 2


What is the type of the ROM chip

Any ROM/RAM according to JEDEC specification for that package can be used with a 3C509B

It's 28 pin connector, please see picture below.

Standard JEDEC DIL I'd say. The B model fits anything from 24 to 28 pin with capacities between 8 KiB (Type xx32) and 64 KiB (Type xx512). Sizes over 16 KiB do need a window setting. Pinout as seen here:

enter image description here

This includes

  • PROM
  • EPROM (usually named 27xx)
  • EEPROM (often named 28xx)
  • FLASH (often named 29Fxx or 28Fxx)
  • Various MRAM

Which one to pick is up to your ability to program and reprogram. The B model does not only support EEPROM as ROM, but as well writing them, thus such might be the preferred solution.

In any case a peek at the 3C5x9B manual might be useful.

and where would you obtain it?

RC.SE is not suited for 'where to buy'-type questions. Pick the type you want to use and hit your preferred search engine with that part number. To find the right one you may try a parametric search at a major distributor first (and compare with its competitors :)).

  • And check the power supply voltage (Vcc pin). 5V (likely) or 3.3V.
    – Grabul
    Commented Aug 1, 2023 at 14:28
  • 4
    It'S 5V- the ISA card (which is seems to be) is still classic PC tech.
    – Raffzahn
    Commented Aug 1, 2023 at 14:57
  • OK, but once he's got it what is he going to do with it? He'll need an Intel hex file containing the boot code and a suitable programmer... Commented Aug 2, 2023 at 6:37
  • @MarkMorganLloyd You may have a look at the site he already pointed to: iPXE - also, that's why I mentioned the need of figuring programming out in the first place, and a possible use of EEPROM which can be programmed while right on the card.
    – Raffzahn
    Commented Aug 2, 2023 at 7:51
  • Yes, I'm planning to burn my custom build of iPXE using Flashrom or Arduino board, whatever seems easiest.
    – Zouppen
    Commented Aug 2, 2023 at 11:53

There may be multiple compatible types for the same pinout so the exact type depends on how large EPROM you need.

The largest possible ROM size is 32k so a matching chip type is 27C256 and compatibles.

  • It's a B, so 64 KiB (xx512) works as well - also, anything above 16 KiB will need window management.
    – Raffzahn
    Commented Aug 1, 2023 at 13:13

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