Lo-tech 2MB EMS Board

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Lo-tech 2MB EMS Board (r02), Assembled

The Lo-tech 2MB EMS Board is an 8-bit ISA expansion card providing up to 2MB of EMS for 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 2MB EMS Board utilises up to four AS6C4008-55SIN 4Mb static RAM chips to provide up to 16Mb (=2MB) total RAM capacity, buffered via a 74ACT245 to ensure compatibility with TTL logic levels, on an 8-bit ISA expansion card. For applications that support EMS, this provides a way to install more than the 1MB total address capacity of the 8086 and 8088 processors.

Following the LIM EMS specification, physical RAM is divided into 16KB pages and addressed via a single 64KB window (the 'page frame') in the upper memory. The pages presented within the window are determined by the value set in four page registers, which are programmed through IO ports. A jumper block enables the page frame IO port base addresses to configured to match the system requirements.

For operation, the EMS board requires a LIM memory manager driver, LTEMM.EXE. This driver provides LIM 4 compatibility.


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..C15 0.1uF X7R Ceramic Capacitor 0603 15 1414610 80-C0603C104K4R
C16 10uF 16V Electrolytic E2-5 1 9451056 140-REA100M1CBK0511P
IC1, IC5 - 74HCT688DW SOIC-20 2 1085321 771-HCT688D652 or 595-CD74HCT688M
IC2 - 74HCT139D SOIC-16 1 1085308 771-74HCT139D-T
IC3 - 74HCT04D SOIC-14 1 1085299 595-SN74HCT04D
IC4 - 74HCT32D SOIC-14 1 1201305 771-74HCT32D-T
IC6..IC9 - 74HCT573D SOIC-20 4 1201309 771-74HCT573D
IC10 - 74HCT245DW SOIC-20 1 1085315 771-74HCT245D
IC11 - 74HCT138D SOIC-16 1 1201307 771-74HCT138D-T
IC12..IC15 - AS6C4008-55SIN 4Mb SRAM SOIC-32 4 1562901 913-AS6C4008-55SIN
JP1 - PINHD-2X4 - 1 1593441 649-77313-101-08LF
RN1..RN3 10k RESISTOR, ARRAY-4, 10K, 1206 1206 3 1770137 652-CAY16-103J4LF

Note: Only one SRAM chip is required to build a functioning board (populate SRAM chips in order from IC12).

ISA Bracket

Uses Lo-tech ISA Slot Bracket Type 3.

JP1: Board Configuration

The board is configured via four jumpers on the pin-header JP1 (O=open, C=closed, position 1 is furthest from IC11 (74LS138D)):

Page Frame Base Address IO Port Base Address
Value (1,2) Base Address
C,C C000h
O,C D000h
C,O E000h
O,O (invalid)
Value (3,4) Base Address
C,C 260-263h
O,C 264-267h
C,O 268-26Bh
O,O 26C-26Fh

See Also