Lo-tech 2MB EMS Board
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:
- R01 - Prototype. See Lo-tech 2MB EMS Board (prototype) Resources.
- R02 - First production release.
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.
Design
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
| 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 | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
- The recommended configuration is E000h and 260h, being C,O,C,C.
- The Lo-tech EMS Board Test Utility can assist with hardware testing after build.