Lo-tech 1MB RAM Board

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Lo-tech 1MB RAM Board (r02), Assembled

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.

This board can be purchased online via the TexElec Store.

Revision History:

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.


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


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

Part Value Device Package Qty Farnell Mouser
C1..C9 0.1uF X7R Ceramic Capacitor 0603 9 1414610 C0603C104K4RACTU
C10 10uF 16V Electrolytic E2-5 1 9451056 140-REA100M1CBK0511P
IC1, IC2 - AS6C4008-55SIN 4Mb SRAM SOIC-32 2 1562901 913-AS6C4008-55SIN
IC3, IC4 - 74HCT138D SOIC-16 2 1201307 771-74HCT138D-T
IC5, IC6 - 74HCT30D SOIC-20 2 1085303 771-HCT30D652
IC7 - 74HCT245DW SOIC-20 1 1085315 771-74HCT245D
IC8 - 74LS05D SOIC-14 1 9592164 595-SN74LS05D
IC9 - 74HCT02D SOIC-14 1 1085298 595-SN74HCT02D
RN1..RN5 10k RESISTOR, ARRAY-4, 10K, 1206 1206 5 1770137 652-CAY16-103J4LF
SW1, SW2 - MULTICOMP MCNDS-08-V DIP-16 2 9471596 774-2068

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.

Switch position ON enables the corresponding page:

Sw1 Sw2
Position Page Memory Range
1.1 0000h 0-64KB (16-64KB when Sw2.8 ON)
1.2 1000h 64-128KB
1.3 2000h 128-192KB
1.4 3000h 192-256KB
1.5 4000h 256-320KB
1.6 5000h 320-384KB
1.7 6000h 384-448KB
1.8 7000h 448-512KB
Position Page Memory Range
2.1 8000h 512-576KB
2.2 9000h 576-640KB
2.3 A000h 640-704KB
2.4 B000h 704-768KB
2.5 C000h 768-832KB
2.6 D000h 832-896KB
2.7 E000h 896-960KB
2.8 - Disables 0-16KB (when Sw1.1 ON)

Note that when used with IBM 5150 16-64KB system board, the system board should be populated with 64KB to avoid bus contention.

See Also