I discovered a new little cutie this time on eBay – a HP4951C.
It is a serial protocol analyser from 1985 which can be used to monitor different serial connection using so-called pods.
These pods can be connected to the main unit and back in the day have been sold separately.
I got the most useful V.24 / RS-232C pod (18179A) with a build-in breakout-box. The only drawback of the unit is the limited speed of 19200 Baud.
And…how is it?
Quite amazing.
My unit was in very good shape, worked (for 3 days – see below) out of the box. My pod cable was missing, but you can simply use a 37-pin-d-sub cables which are 1-1 connected (you just have to pull out pin 34).
There are several sites which have a ton of information about this device, so no need to repeat this here. I was able to create some of the software disks by using greaseweasle and a modern 3.5″ floppy drive.
So I have a terminal emulator on the device as well.
I managed to connect to the device remotely from an external PC/Mac so that I could use the super-ancient HP DOS tools 5XREMOTE.EXE to upload and download stuff.
What about the three days?
While doing some floppy drive maintenance, some of the RIFA caps in the device exploded as they always do after 40 years.
Since I have to take the whole device now apart anyway, I decided to do a full renovation now.
This means exchanging all caps on the main board, replacing the on-board NiCd battery with a new one (to avoid future leaking incidents – see a replacement here).
Tips so far
Floppy Drives
The HP 4951C uses a 3.5″ double density drive. You can use HD disks and an HD drive to create disk for this…but when writing the disk, you have to cover the HD hole on the disk.
Otherwise the disk will be written half-correct but fail on verification.
Greaseweasel can directly write the TD0 (Teledisk) images to a disk, so no need to convert something.
Remote Connection (Cable)
The remote connection is very picky when it comes down to the handshake signals. You strictly have to follow this here (copy from the HP Operations Manual):
The HP 4951C is configured as a DTE for remote operations. If two units are connected directly without modems, one unit must be configured as a DCE. For applications with no modem, use a modem eliminator cable such as the RS-232C/V.24 printercable M/M (HP # I 3242G). You may also open all the breakout switches except pin 1 on one of the pods, and jumper the following pins: 2 to 3, 4 to 8, and 5 to 6 and 20.
Remote Connection (Tool)
It is a bit tricky, but once installed pretty reliable:
- Install DOSBOX on your favourite PC/Mac
- Create a local directory on your PC/Mac and mount this in DOSBOX as a drive – copy the HP TOOLs to this directory
[autoexec]
mount C: /Users/thagemann/hp-tools/ - Add a config file connecting one of your serial ports directly to the virtual DOSBOX environment.
[serial]
serial1=directserial realport:tty.usbserial-10 - Then you can start 5XREMOTE.EXE and connect to the 4951C
What’s next?
As soon as the spare parts arrive, I will put everything back together and check if I broke something. Of course, will be documented here.
In the meantime I started to hack the system firmware a bit – maybe I can reach the goal of writing apps for this device too.
There are some custom apps for the 4952A, but they do not work on the 4951C – completely different architecture.
I started to document memory map, I/O map and a lot of other stuff and reverse-engeneer the boot rom – let’s see how far I can get.
I also dumped the ROMs and will provide them on GitHub – they have version 5 of the firmware. The only ROMs I found during my research are an older version 4.