HP 4957A


HP 4957A

The HP 4957A is a serial protocol analyzer. It is the third generation of similar looking machines, starting with the 4951A in 1985, then the 4952A in 1988, then finally the 4957A in 1992. They are all neat, but The 57 is my favorite of the series because it has the integrated RS-232 test pod and the full 80 column VT100 emulation. However it is harder to repair since it is undocumented and full of Xilinx FPGAs. The earlier machines are partially documented and have mostly standard logic.

This machine harks back to the pre-Ethernet time when data networks used serial packet oriented links, like SDLC, HDLC or BiSync. It is a data network analyzer for such protocols. But it also supports asynchronous character oriented serial RS-232 links, and thus is still quite helpful today for more mundane serial link debugging.

Another great capability is the full VT100 emulation. Unfortunately that's what was broken on our unit (in addition to the power supply and the backup battery), but we went around it with a ROM swap, see video.

HP 4957A Videos

In this video, we repair the power supply, build a battery replacement, and fix the VT100 emulation high resolution screen glitch with a ROM swap.



Our two units had different ROMs. One was the A0001 version, dated 1991, which had the low resolution (40 columns) VT100 emulation. The other had the A0002 version, dated 1993, which had the high resolution (80 columns) VT 100 emulation. We dumped the 5 ROMs that differed of each machine. A picture of where they go in the board is also provided.

The ROMs in the A0001 version were AM27C010.

The ROMs in the A0002 version were a combo of one AM27C010 and three ST M27C1001. The type is indicated in the title of the dump files.

Board with A0001 ROMs (40 column VT100 emulation)

Board with A0002 ROMs (80 Column VT100 emulation)

ROM dumps

HP 495X Application Diskettes

Important: HP 495X diskette set recreation and copy protection

The HP 4957A comes with a set of diskettes with some applications, simulation menus and example data on 3.5" diskettes, which considerably expand the capability of the machine. But these are not regular PC formatted diskettes: they are HP LIF formatted, and therefore not simply copyable on a modern PC. To make things worse, HP implemented a copy protection scheme that is specific to the HP 495X series. You can only copy the HP 495X diskettes using an HP 495X, at least in theory.

In practice, to go around the non-PC format and HP 495X copy protection, use the method below to recreate working HP 495X diskettes from the .imd files I provide.

1- Format a 3.5" low density diskette in the HP 495X.

2. Use the ImageDisk software on a vintage DOS machine with a 3.5" drive (USB drives on a modern OS won't work) to write the diskette you formatted in step one from the image files below.

3- This does not apply to the 495X PC Utils diskettes which are standard PC diskettes, containing utility .EXE programs that run under DOS. You can skip the formatting and recreate them straight from ImageDisk onto blank diskettes.

HP 4957A Standard Diskette Set

HP 495X Optional Application Diskettes

HP 4951C Terminal Emulator Application

In the HP 4951C, the VT100 terminal emulation is an external application you have to load from this diskette. This app is not compatible with the HP 4957A, which instead has it available for loading directly from ROM, and has full 80 character per line resolution (at least in the later revision of the ROMs, as we learned in our video).

Hacking the HP 495X!

Some really clever chaps have figured out how to partially reverse engineer the HP 495X and write new applications that work on it. They managed to get Z80 Basic kinda-sorta working and to write the "Snek" game! See video and links below. I have not dwelved into this yet.

YouTuber asr33 wrote a game for the HP495X!

His source code repository is here:


He based his work on the discoveries of David Kuder, see below.

Asr33 is also the author of the mind-blowing awesome ASR33 emulator software, the best thing besides owning a real ASR33:


And Trevor Jahanssen Aase is trying to port CP/M to the HP 4952, a work in progress at this point: