GITHUB: https://github.com/hkzlab/ISA_EMS_2Mb

This is a reimplementation of the lo-tech.co.uk 2Mb ISA EMS board with the following changes:

All thru-hole instead of SMD, while retaining a rather compact format

Activity led to indicate when the memory is accessed

Disclaimer

I take NO responsibility for what happens if you decide to build and use this card. Your computer might crash, catch fire or be destroyed in other nasty ways. Also, as this card is mostly based on the work of others, You're encourauged to take what you deem fit from this, and use it in your projects!

Additionally, do NOT CONTACT lo-tech.co.uk FOR SUPPORT, while this board is derived from their work, it is not their product, please do not waste their time.

Configuration

EMS

To configure the EMS, jumpers on J2 header are used.

Pins 1 and 2 configure the Page Frame base address, while 3 and 4 configure the IO Port base address.

EMS Page Frame Base

1 | 2 | Base |

---+---+-------+

X | X | C000h |

---+---+-------+

| X | D000h |

---+---+-------+

X | | E000h |

---+---+-------+

| | Inval |

---+---+-------+

EMS IO Port Base

3 | 4 | Base |

---+---+------------+

X | X | 260-263h |

---+---+------------+

| X | 264-267h |

---+---+------------+

X | | 268-26Bh |

---+---+------------+

| | 26C-26Fh |

---+---+------------+

Usage

You need a memory manager to use this board. I'm including the LTEMM driver provided by lo-tech in this repository. Please refer to this page regarding the terms of use.

Bill of Materials

All ICs are in DIP package.

U7, U8: 74HCT688

U3, U4, U5, U6: AS6C4008 (You don't need to fill all sockets: fit as much memory as you need starting from socket 1!)

U11: 74HC04 (HCT is fine too)

U9: 74HCT139

U10: 74HCT32

U12: 74LS245 (You can substitude this for an HCT/ACT/F/ALS part)

U17: 74HCT138

U18, U19, U20, U21: 74HCT573

RN2: res. network, 10k, bussed, 5 pins

RN3: res. network, 10k, bussed, 9 pins.

R3: ~470-1k resistor (depends on the type of LED mounted on D1 and desired brightness)

D1: 3.0mm or 5.0mm LED, you can use an horizontal mount led too

J2 2x4 2.5mm breakable header

C5, C6: 10uF / 50v electrolytic caps

Remaining caps: 16x 100nF, ceramic

ISA bracket: Keystone 9202, plus screws. You will have to cut a hole for the LED.

Known Issues

I have received reports of this board being unstable when used with some homebrew PC clones, reports included the Xi8088 and NuXT. I have noted similar issues on a Micro8088. On the other hand, an all-CMOS board was tested fine on multiple AT clones I have. Replacing the following ICs with ones from the TTL family solved the issue for me: U7, U8, U9, U10, U11, U12.

Note that, for increased stability, I use an ISA terminator with these homebrew computers.

Credits

Thanks to lo-tech.co.uk for having published their schematics

Thanks to Sergey for all his projects!