Lo-tech 8-bit ROM Board

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8bit-rom-board-assembled.jpg

Note: This board has been superseded by the Lo-tech ISA ROM Board.

The lo-tech 8-bit ROM Board is a simple and cheap to make (both in terms of the PCB and the components) 8-bit ISA board that provides a 32 or 64KB ROM via a flash chip, which can be (re)programmed a practically limitless number of times directly within the host PC via the lo-tech XT-CF flash utility.

The board can be used to store any ROM image (or images) in any PC with an ISA slot. The DIP switches allow an address base selectable between 8000h and F800h and configuration as either 32KB or 64KB.

The board uses entirely through-hole components for simple home assembly.

Contents

PCB

8bit-rom-board-rev2.jpg

The rev.2 PCB is almost identical to the original version, with just a few trace routing changes. Both have the same BoM.

Boardhouse Requirements

  • 8 mils clearances and minimum track width
  • 0.4mm minimum hole size
  • Can be manufactured on a 50x100mm board
  • Gold fingers are preferable but not required

Assembly Notes

  • Thoroughly wash the PCB before assembly with Isopropynol
  • Generally, work from the smallest components to the largest, so that they are held in place by the work-surface when soldering
  • The use of syringe flux will make it easier to get good joints
  • When soldering multi-pin components (such as the IC sockets), solder one pin (only) then check the physical placement before continuing, as adjustment later is difficult
  • Guideline order of assembly:
    • Resistors R1 & R2
    • Capacitors C2 & C3
    • IC sockets (note orientation via notch - align to silkscreen)
    • Resistor network RN1 (datasheet) - mount so the end with the dot (pin 1) is nearest to the ISA edge connector
    • DIP switch (note orientation to board photo, to ensure switch numbers tally to configuration settings below)
    • Electrolytic capacitor C1 (note orientation!)
  • The DIP switch supplied in the kit is washable, so the board can be thoroughly cleaned to remove flux residues with a toothbrush in isopropynol once fully assembled.
  • Fit the to ICs in the sockets, noting orientation via the notch (also refer to the constructed board photo). The IC pins will need to be bent to 90° - with the IC on it's side with a row of pins against a smooth work-surface, gently push down and twist the body until such alignment is achieved

Bill of Materials

Mouser Part No Description Quantity
804-39SF010A7CPHE SST39SF0x0A or AMIC A29010 Flash (DIP32) 1
21RZ310-RC Ceramic Capacitors 0.1uF Z5U 20% 2.5mm L/S 2
140-REA100M1EBK0511P Electrolytic Capacitor 10uF 20% 5x11mm 1
774-2068ST DIP Switch SPST 8-way 1
571-1-390263-2 32P DIP Socket 1
571-1-390261-6 20P DIP Socket 1
595-SN74F521N SN74F521N Comparitor 1
652-4607X-1LF-10K Resistor Network 7 pins 10Kohms Bussed 1
279-CFR25J10K Carbon Film Resistor 10KOhm 1/3W 2
  • Board design permits the use of a range of DIP-32 flash chips or 1, 2 or 4Mb:
  • Regardless of the chip used, the first 32KB or 64KB (only) will be mapped into the PC address space
  • Base address configuration per DIP Switch Settings

DIP Switch Settings

Summary

Switch Function On Off
1 ROM Write Enable Enabled Disabled (ROM cannot be erased or written)
2 ROM Enable Enabled Disabled
3 ROM Base Address A15 Low High
4 ROM Base Address A16 Low High
5 ROM Base Address A17 Low High
6 ROM Base Address A18 Low High
7 Flash Chip A15 Source ISA Bus A15 Low
8 Comparitor A15 Source ISA Bus A15 Low
  • For 32KB operation, set 3 as required, 7 Off and 8 On
  • For 64KB operation, set 3 On, 7 On and 8 Off

32KB Address Mapping

Base Address Sw3 Sw4 Sw5 Sw6 Sw7 Sw8
8000h* On On On On Off On
8800h* Off On On On Off On
9000h* On Off On On Off On
9800h* Off Off On On Off On
A000h On On Off On Off On
A800h Off On Off On Off On
B000h On Off Off On Off On
B800h Off Off Off On Off On
C000h On On On Off Off On
C800h Off On On Off Off On
D000h On Off On Off Off On
D800h Off Off On Off Off On
E000h On On Off Off Off On
E800h Off On Off Off Off On
F000h On Off Off Off Off On
F800h Off Off Off Off Off On

* Note that these ranges aren't valid for option ROMs in PC architecture.

64KB Address Mapping

Base Address Sw3 Sw4 Sw5 Sw6 Sw7 Sw8
8000h* On On On On On Off
9000h* On Off On On On Off
A000h On On Off On On Off
B000h On Off Off On On Off
C000h On On On Off On Off
D000h On Off On Off On Off
E000h On On Off Off On Off
F000h On Off Off Off On Off

* Note that these ranges aren't valid for option ROMs in PC architecture.

Sample ROM Image Files

8-bit IDE

A customised build of the XTIDE Universal BIOS (based on build r443) to enable CompactFlash media to be used connected to a standard 16-bit ISA IDE controller but installed in an 8-bit XT-class machine. The IDE controller must be using standard ports 1F0h and 3F0h.

Testing with with Diamond Flower MIO-400 KF REV F and Goldstar Prime 2 controllers, and expected to work with any standard IDE controller using the specified port addresses.

See http://code.google.com/p/xtideuniversalbios for official builds of the XTIDE Universal BIOS and source code.

High-Density Floppy

XT compatible BIOS images to run high-density floppy drives, for example from 16-bit ISA multi-IO cards.

See Also