All the 8-bit machines are derived from the same base: 1MHz 6809 CPU, at least one 6821 PIA, and a custom video circuit. There are, however, several important differences in the actual implementation of this.

There are two ranges of machines called TO and MO. Originally the MO5 was supposed to be a low-cost alternative to the TO7. To achieve this, the timer PIA was removed, the memory map simplified, and most of the video logic integrated in a gate array. However, it turned out the MO5 is actually a quite capable machine and does not feel really low-cost compared to the TO7.

The lack of compatibility and apparent similarity of the TO7 and MO5 led to rather bad reception. From then on, Thomson will be very careful not to break compatibility again, in an attempt to consolidate its small user community. However, the following machines will make a lot of changes to the hardware, extending the use of gate arrays and eventually completely redesigning the video logic to make better use of modern components and further simplify the hardware.

The result of this is that the later machines often have two or more ways to do the same thing, one being legacy and having some limitations, and the other more flexible and generic. In some cases, work previously handled by the PIAs is actually emulated by the custom gate array, with the actual PIA pins left unused.

On the software side, machines come with a “monitor” ROM implementing the base OS and a built-in BASIC. The monitor is different for each machine (and for some there are also several revisions of it). The BASIC is available in several versions as well, with the latest machines providing 2 or 3 of them. Some versions of the BASIC add disk access commands, others extend the capacities with the use of RAM banks.

One thing to mention is that the monitor always handles interrupts directly in its ROM. While most other machines feature a “RAM only” mode, there is no such thing on Thomson ones, and the interrupt handler will always execute from the internal ROM. This makes the use of the Timer PIA a lot less interesting than it could be.

A feature common to the whole range is the availability of a light pen, but starting from the TO9 it became possible to also use a mouse.

On the performance side, the Thomson machines are ok. The 6809 runs at 1MHz, faster than the PAL version of the C64, and it is a more powerful CPU, with more registers. However, the video and especially the sound hardware are rather limited (you get a buzzer, or a 6-bit DAC, but no DMA). Generating sounds with these use a lot more CPU time than what you can handle with a dedicated sound generator chip.

On the video size, all video modes revolves around various uses of a 16K video RAM split in two banks. The screen size is fixed, and there is no hardware scrolling, sprites, or anything like that. Things get a bit more flexible starting with the TO9, which introduces several new video modes, 60Hz video support, and the possibility to switch between 4 different video pages.

Here is a rough “equivalence map” of what Thomson was trying to compete with for each released machine: