IBM 5100

IBM 5100 (1975)

Since the initial price of this computer was between 9 and 20 thousand dollars in 1975 (43 to 95 thousand dollars in 2018!), I didn’t expect to find it in a small town near the even smaller town where I live. A friend of my father-in-law gave him this IBM 5100 for free, probably because my father-in-law talked to his friend about my passion. This person ran a small repair shop, but sadly passed away shortly after and I never got the chance to ask him where this computer came from.

IBM 5100

Serial number 13691.

IBM 5100 serial number

This IBM 5100 was completely dead when I took it home; with some help from a friend I found that a big line filter under the monitor was shorted.

IBM 5100 line filter

After bypassing it, the 5100 was at least showing some signs of life. By removing one board at a time, I isolated a faulty one and got some garbage on the screen. Sadly there are little chances that this computer will be fully repaired – most of the boards’ components are proprietary and undocumented.

The 5100 was one of the first portable computers: it had the central unit, keyboard, monitor and mass storage in a single small case (small for its time, of course). I won’t go into the technical details of the machine as there’s plenty of information about the IBM 5100 on the net.

I disassembled the computer to clean the various parts; the underneath of the keyboard was covered with a sheet of plastic material that 40 years ago was probably supposed to protect it from dust or liquid spill, but in time it became dry and friable.

IBM 5100 keyboard

Here’s the keyboard pcb:

IBM 5100 keyboard

And this is the top part:

IBM 5100 keyboard

A detail of the keys:

IBM 5100 keyboard

And the final result:

IBM 5100 keyboard

The small 5″ black and white CRT display:

IBM 5100 monitor

The internal tape unit:

IBM 5100 tape drive

Unlike modern PCs, every additional board had to be placed in the right slot. The IBM 5100 was sold in different configurations, and my system appears to be one of the most complete: all the slots are populated, with both APL and BASIC languages and the full 64KB of RAM – “Read/Write Storage” in IBM terms.

Below are the empty card slots of the IBM 5100, and after that all the cards as named in the manual.

IBM 5100 boards

Slot A –  I/O Cable Driver

IBM 5100 card A - I/O Cable Driver

Slot B – Expansion Feature

IBM 5100 card B - Expansion Feature

Slot C – BASIC ROS (left) and APL ROS 3 (right)

IBM 5100 slot C - BASIC ROS and APL ROS 3

Slot D – APL ROS 2 (left) and APL ROS 1 (right)

IBM 5100 slot D - APL ROS 2 and APL ROS 1

Slot E – ROS Control

IBM 5100 slot E - ROS Control

Slot F – Base I/O

IBM 5100 slot F - Base I/O

Slot G – Controller

IBM 5100 slot G - Controller

Slot H – APL Supervisor Executable ROS (left); BASIC, I/O, and Diagnostics Executable ROS (right)

IBM 5100 slot H - APL Supervisor Executable ROS / BASIC, I/O, and Diagnostics Executable ROS

Slot J – Display

IBM 5100 slot J - Display

Slots K, L, M and N – Read/Write Storage (16KB of RAM each)

IBM 5100 slot K, L, M, N - Read/Write Storage

After the top cover is removed, the 5100 has a hinge on the right and it can be opened to easily access the system. Here’s a picture with all the boards in place:

IBM 5100 boards

Today we are used to icons and graphic symbols to describe every connector or functionality of the computer: we all recognise the USB or the video icons, the wireless symbol, and so on. In 1975 computers weren’t so common, so IBM preferred to localise as many elements as possible, including button or LED labels: my model is the Italian one, and IBM translated every word including for example “reset” (we use the word in English nowadays) that became something like “start again”.

IBM 5100

The manuals that I have for this machine are:
BASIC Introduction
BASIC Reference Manual
APL Introduction
APL Reference Manual
Maintenance Library Manual
User’s Catalog

IBM 5100 manuals

Some of the manuals include tapes.

IBM 5100 APL Introduction manual

All the items were left in a cellar for years – probably decades – and they were quite dirty. The covers of the manuals were full of mould, but luckily the internal pages were still in a decent condition.

IBM 5100 BASIC Introduction manual (dirty)

There’s a long plastic sheet that slips out from the top cover; it briefly explains all the error codes from the system.

IBM 5100 error codes

Small for its time but fairly hefty by today’s portability standards.

IBM 5100

I made the top cover transparent with two shots on a tripod and some Photoshop wizardry.

IBM 5100

Here’s a last picture of the IBM 5100 before putting this historic machine back on the shelf. One of the lower shelves, since it weighs around 25 Kg.

IBM 5100

5 thoughts on “IBM 5100 (1975)

  1. Chris

    that’s super … never seen one of those !

    looks to be broadly similar format to an HP80, which we almost bought a few years later … which was a predecessor to an HP Integral, which we also considered investing in – but too expensive, of course !

    (Lancashire, UK)

    1. Giacomo Vernoni Post author

      Hi Frank, I checked my images but I didn’t shoot any picture of the back of the boards. Anyway the back is like the front but without any IC: a regular grid of dots with some traces between them.


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