S~100

S-100 Expansion for Nascom 1

This is a shortened version of an article sent in by Mr. K. P. Curtis of New Barnet. The INMC would very much like to hear from anyone else who has expanded their Nascom via the S100 bus.

I decided to expand my Nascom 1 with a S100 expansion kit currently being offered. Despite being more expensive in the short term, I felt that it widened the choice available to further enhance my system.

The kit consisted of :–

1. A double sided PCB without plated through holes that will accomodate three cards.
2. Two edge connectors and ribbon cable to connect the Nascom to the expansion board.
3. All required 1C’s and sockets except one, a tri-state buffer for control signals which is optional. (Would recommend this is used ED.)
4. Lots of vero pins to make the through board connections.
5. Decoupling capacitors.
6. A 2K Basic interpreter (B-Basic).

Assembly

The instruction for assembling the board are adequate but the following points should be noted:

Insert all vero pins before soldering and very carefully check their positions as it is very easy to get them in the wrong positions.

Some pins need to be cut short to allow sockets to sit flush on the board.

The longest part of assembly was making up the ribbon cable assembly – very tedious.

Documentation

A circuit diagram of the logic is provided but does not match the board layout and the logic that generates the MEMEXT signal, although functionally the same, is misleading. Also only the pin numbers of the buffers are given and some of the IC legends appear to have been incorrectly printed on the board.

Beside these errors I found two connections not made in the external I/O logic : IC5/15 to IC10/1 and IC9/11 to IC9/10.

Power Supply

The ready built 8K RAM card that is being used requires an 8V unregulated supply from the S100 bus. This may be obtained from the existing power supply – after the rectifier but before the dropper resistors and regulator. Before attempting to use the kit and memory though, the rating of the rectifier should be checked – mine lasted for