ABC-24 / 26 Technology
The ABC 24 and 26 machines are very advanced and sophisticated machines by the standards of the late 1970s.
Extensive use is made of advanced LSI ICs and these are used not just for specific interface functions but also for functions such as DRAM control and display generation. These latter functions could be done in software but Ai’s use of hardware would have provided much greater performance, at the cost of increased hardware.
CPU and Memory
The machines are based on a Z80A CPU running at 4 Mhz.
- 64k bytes of RAM is provided by 4 banks of 8 4116 DRAM ICs with an 8202A DRAM controller
- There is a ‘bus’ connector on the main board and the sales brochures suggest that further memory expansion was possible.
The integrated CRT display was driven by an 8275 CRT controller chip with EPROM character generator. This hardware would allow high display performance and customisation of displayed character sets while preserving CPU resources for user tasks.
The keyboard is hard wired and uses a hardware scan and encode chip, again using hardware to reduce the need for the CPU to do basic housekeeping tasks.
- Dual floppy drives were built in
- 8-inch for ABC 26
- 5.25-inch for ABC 24
- A hard disk option was also available.
A most unusual item is the AM9511A Maths Co-processor. Dedicated maths chips became familiar in the IBM PC era but were very uncommon before that time. Ai’s use of a maths processor suggests that they had high ambitions for the ABC series machines.
The sales literature indicates that a double-precision coprocessor was available as an option.
Two serial and one parallel peripheral ports are a useful offering and these are supported by dedicated interface ICs. Less common is the provision of a GPIB interface with MC 68488 GPIB controller IC and four 3448 GPIB driver ICs.
The entire logic is contained on one large board that is constructed by the “multiwire” method. This unusual construction method involved machine-laying insulated copper wires onto a sticky base. Wires could cross each other as required until the entire circuit was built up. When the wire laying was complete a cover layer was applied and the resulting sandwich was permanently bonded together. Holes were then precision-drilled to pierce the inner conductors and were plated-through with metal to establish contact with the cut ends of the conductors. ICs and other components were then loaded into these holes and soldered, completing the circuit.
Multiwire seems to have been used particularly for making prototypes and smaller runs of complex boards but the method was superseded by advances in multilayer board design and manufacture.
The multiwire method is a great frustration for restorers because the circuit is hidden and quite untraceable. Repairs to such boards are almost impossible, except by flying leads to bypass defects.
Through the magic of the interwebs and the generosity of a fellow collector, circuits and technical documents were discovered for these ABC machines. Copies are available on the Ai Data pages. Further technical information and any software will be gratefully received.
The ABC machines are housed in very solid and nicely made metal cases. The same quality of design and manufacture that was put into the electronics has been put into the physical casing.