A 1970s School 'Computer'

Originally posted on the RetroMat website, but fits better here.

Programming at school in the decade before the microcomputer revolution.

Diehl Combitron S with paper tape peripherals - a machine of similar type to the one we used.
Picture courtesy of technikum29.

In around 1973 or '74, at Wellsway School in Keynsham, our Maths class was introduced by our wonderful teacher, Mr. Grace, to a machine like the one above. It was shared between three schools in the area, one term at a time. While it was referred to as a 'computer', it was really a desktop calculator with some programming capability. We had taken Maths O-level a year early (a reflection of the SMP syllabus) and in the fifth form we had a little more time on our hands for such things. Programming was a new experience for us. Indeed, when we were first introduced to the machine, simply using a calculator would have been novel, with pocket calculators being a recent development, hence expensive and comparatively rare.

Programs were worked out in advance and written down by hand on paper. Then when we had our allotted sessions on the machine, we'd key them in and try to get them to run. If we were lucky, we'd correct them there and then. Either way, they would then be saved onto punched paper tape (possibly manually). As this was a shared machine, getting a program to run successfully was time-consuming and I remember getting up early on more than one occasion to get some time on the machine.

I recall that programs comprised up to 100 programming steps*, contained in 10 program registers of 10 steps each. If you needed to continue after 10 steps, you would go to the next program register. Perhaps we could implement subroutines**, maybe without using that term, using these program registers? In addition, there were 10 registers available for storing numbers. It was possible to repeat steps by jumping back to the beginning of a section of code and - crucially - to execute code depending on some condition being met. This last features provided the ability to extend programming beyond being simply a series of automated steps on the calculator (keystroke programming).

As to the device itself, I had long forgotten the name. However,  I did recall the distinctive symbols used for sending and retrieving data to and from registers, like this  -  V  - for one such operation and the same symbol but upside down for the other operation.  After a lot of searching, I found that these symbols seem to be unique to German manufacturer Diehl, and thanks to information about the range of their calculators on the very extensive CALCUSEUM site, I think our machine was either the Combitron-S, shown in  the centre of the picture above, or a similar machine, the Combitronic, these devices being limited to 10 program registers of 10 steps each. However, neither the manufacturer's name nor the names of the machines sound at all familiar, so I wonder if it could have been re-badged with a different company's name?

Assuming I'm right, then our machine dated from the late 60s. As I wasn't aware of its existence before 1973, I wonder if it might have been donated by an organization replacing it with a more recent machine?

(BTW, the tape puncher at the left in the above picture seems to have been an automatic one that punched as you typed on the main machine. I don't recall using one like that - it was likely this manual tape puncher here that we had).

Assuming it was this machine, it was pretty basic - no stored values for π or e and no trig. functions. (Even my first pocket calculator, bought a year or so later, had functions that weren't available on it!)  However, there's a sample program (via one of the links below) that calculates e(x) using a Taylor series in just 4 program registers, using 2 memory registers. No doubt sin (x) or cos (x) could be done similarly. 

It was also quite clunky to use - requiring numbers to be moved in and out of the central unit, to and from registers. While cumbersome, it was however a great introduction to programming in machine language for me, specifically in z80 Assembly Language for the Sinclair Spectrum some ten+ years later!

Set exercises included e.g. calculating the volume of a sphere based on a radius input by the user (π would have been assigned to a store by the programmer), playing the game of NIM against the machine and simulating a solution to the Tower of Hanoi puzzle. That first example would have used simple keystroke programming, the last two would have used logic. My code is long gone, but I was inspired to write NIM for the Sinclair Spectrum in recent years!

After completing these challenges, I decided to set my own by using an approximation method I had recently learnt about to determine the square root of a number. Never mind that the 'computer' came with its own, much faster, square root function, nor that there were likely many more suitable candidates for calculating a function! It was great fun and quite satisfying to do. The program looped to determine successive approximations of the root and was supposed to stop when they were the same. However, sometimes it continued to run with the printed approximations oscillating between two values and the program had to be stopped manually. Again, the code is long gone, but here is my Sinclair Spectrum version.

An enterprising fellow pupil used the machine to generate artillery tables and these were displayed in a chart along with the relevant equation at a stand at an evening event for parents of prospective pupils. Trigonometric functions would have been needed for this, so he must have coded his own!

By the time I left the school in 1976, the machine seemed very dated. In fact, I remember the Head of Maths, Mr. Batey, getting a programmable pocket calculator not long before I left and our machine must surely have been ditched soon after!

(Not that I was aware at the time, but Diehl had been active in updating their range - the Alphatronic from 1973 was much more powerful, could be connected to peripherals with magnetic media, had alphabetic keys (presumably so that text messages could now be displayed in programs), and 'proper' programming, including keys for GOTO, GOSUB and LABEL. Later models were even referred to as 'computers' by Diehl. In the late 1970s, the company released a mini-computer range, the DS2000 and DS3000, with integrated CRT display, aimed at the technical and scientific market, but it seems they left the business soon afterwards).

It would be great to see one of these machines working again. And I wonder - how difficult would it be to create an emulator?

(Almost) finally - if any fellow ex-pupils or ex-staff (!) at Wellsway, or one of the other schools in the area, remember this machine, I'd be delighted to hear from you!

Foot notes

* actually it's just 10 characters per program register, so even more limited than I had remembered!

** I was wrong about subroutines, if our machine was indeed a Combitron-S or Combitronic. Having reviewed the sample programs (see below), it seems that while you can jump between program registers, there is no hierarchy and you cannot return control to a calling program register.

More information

• For sample programs for a similar model, albeit with with less powerful programming capability, the Diehl Deltronic P, see the links to the brochure pages (in German) here
• To make sense of the syntax used to perform calculations, it may help to view this schematic (in French) for the Diehl Sigmatronic calculator (not programmable as far as I can tell, but the calculator part seems to be the same). In particular, note that there are separate registers for addition/subtraction, multiplication/division (as well as for accumulation). 
• For more information on the range of Diehl electronic calculators, see here and this document (in German) here
• For a closer look at one of the sample programs above, and the calculator syntax, see this document for some comments I have added
• Summary of symbols used by Diehl electronic calculators (focus on Combitron-S, Combitronic)
• An attempt to understand the format of the punched paper tape. (If only I had kept my programs then it should have been possible to decipher them simply by looking at the locations of the punched holes!)