The Lo-tech 1MB RAM Board is an 8-bit ISA expansion card providing up to 1MB of system RAM to IBM compatible PCs. The small form factor makes the board suitable for the IBM Portable Computer 5155.

Revision History:

• R01 — Prototype. See Lo-tech 1MB RAM Board (prototype) Resources.
• R02 — First production release.

Technical Description

The Lo-tech 1MB RAM Board utilises two AS6C4008-55SIN 4Mb static RAM chips to provide 8Mb (=1MB) total RAM capacity, buffered via a 74HCT245 to ensure compatibility with TTL logic levels, on an 8-bit ISA expansion card.

DIP switches enable each 64KB physical page to be independently enabled, so making possible RAM expansion of all systems to the maximum 640KB, regardless of the size of the system board RAM fitted. To extend compatibility further, the first 16KB can be excluded, so providing compatibility with the IBM Personal Computer 5150 when the system board is populated with the minimum 16KB RAM.

The board can also provide upper memory (between 640KB and 1MB) that, whilst not generally used by MS-DOS in an XT class machine, can be used for specific purposes such as TSRs, disk cache and print spoolers. Any complete unused physical pages can be populated by the RAM board for this purpose.

Design

• AS6C4008-55SIN 4Mb static RAM datasheet

Use of this design is provided subject to the lo-tech.co.uk Terms and Conditions. Copyright (c) 2013,2014 Peacon Ltd.

Assembly

This PCB makes use of SMT components:

• SOIC chips have a pitch of 1.27mm
• 0603 package capacitors and resistors

Assembly requires flux and a temperature controlled soldering station.

Bill of Materials

ISA Bracket

Uses Lo-tech ISA Slot Bracket Type 3.

DIP Switch Settings

The board is configured via two banks of DIP switches, which control whether each physical segment (=64KB) is active on the memory board.

JP1, when closed, excludes the first 16KB to provide compatibility with the IBM Personal Computer 5150 when the system board is populated with the minimum 16KB RAM. In this way, system memory can be expanded to the full 640KB (plus upper memory).

Switch position ON enables the corresponding page: