System 1 Trainer

System 1 Trainer Front View Acorn

I was asked by the Centre for Computing History if I could make them some replica Acorn System 1 Computers for use on their educational visits however there are a number of difficulties with building replica System 1's in any quantity.  The 'clicker' keyboard can be replaced with tactile switches but the displays are difficult to obtain (TEXIT calculators are about the only source) and the ribbon cable between the two boards is fiddly to wire up.  So I suggested an alternative approach - a single board solution similar in size to a MK14.

The System 1 Trainer is a single board version of the original Acorn System 1 6502 CPU Board with the Cassette Interface, the circuit is essentially the same with a few changes:

The board is 100% software compatible with the original System 1 (except for the lack of 2nd 8154).

System 1 Trainer Top View

The board has an onboard 5V regulator and links allowing for powering from either an external 7 to 9V source (fit LK6) or directly from 5V (fit LK5) e.g. a USB power source or power bank.

A DIN socket can be fitted for the Cassette Interface (PL1b).

The crystal oscillator has been reconfigured to use a more widely available HC49u series mode crystal.

The EPROM is loaded with the original Acorn Monitor, because this is only 512 bytes then I have provided a build for a 4K EPROM which includes the applications listed in the Acorn System 1 User Manual and Teletext Board Technical Manual with some other useful programs.

Rack Mount Configuration

The board is dual purpose and can be cut down and used as a plug-in Eurocard sized 6502 processor card with cassette interface so forming the basis of an Acorn System 2.

In this configuration a 20-way connector can be fitted for the keyboard and the 2nd DIN socket position (PL1a) used for the Cassette Interface.  A link (LK3) allows power to be derived from the backplane.

The EPROM is then programmed with Acorn's COS Cassette Operating System.

Using an HM6116 SRAM (IC3) seems to work fine when the board is configured as a 'Trainer' however when used in the cut down rack mounted board I and others have found that '6116' SRAMs don't always behave reliably and recommend using an NEC uPD446C-3 in its place which seems to work fine (yes the photo still shows a 6116 !).

System 1 Trainer Rack Mount

Schematic

 

The original Acorn component references have been used (the Cassette Interface components are 100+).

Bill Of Materials

Front Panel Drilling

 

The PCB is available (for sale) or you can purchase a complete kit, to support this I have added a Building a System 1 Trainer page.

 

Here is what's included in the kit:

 

 

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