Lo-tech ISA USB Adapter

From Lo-tech Wiki
Jump to navigation Jump to search
Lo-tech ISA USB Adapter PCB

The Lo-tech ISA USB Adapter is a bootable storage adapter for IBM PC, PC/XT, PC/AT and compatible hardware - essentially any PC with an ISA slot, enabling a standard USB flash drive to be used. This eliminates the dependence on ageing hard disks and floppy disks, and makes data exchange with a modern PC easy. The design is based on the CH375 USB chip and is a development of the Lo-tech ISA CompactFlash Adapter revision 2b.

The BIOS ROM used by the card (available here) enable the host PC to boot from storage presented by the adapter, which appears in DOS as an ordinary fixed disk (i.e., drive C, D, etc). No DOS drivers are needed. Note that the maximum partition size under MS-DOS is 2GB, meaning a total of 8GB can be accessed and the current BIOS provides up to 504MB. For users not requiring ROM boot, a CONFIG.SYS driver will also be available soon.

The PCB itself is a 4-layer, small form factor 8-bit ISA adapter. Building this PCB requires some surface-mount (SMT) components, the smallest being 0603 capacitors. Mounting holes are provided for a custom ISA slot bracket, which can be 3D printed in plastic at home, or from commercial vendors in various metals.

The design provides:

  • Bootable storage adapter for 8-bit PCs
  • Small form-factor, to enable use in machines with limited expansion slot space such as the Tandy 1000HX, Sinclair PC200 and Amstrad PC-20
  • IBM Personal Computer XT System Board Slot 8 compatibility
  • Selectable BIOS address - C800h or D800h

The BIOS is provided through an in-system re-programmable 32KB flash-based ROM, and since the BIOS is only 8KB, 24KB is available for other purposes, and is byte-programmable - the board can therefore function as a universal ROM board.

The board uses the CH375 USB interface and provides optimised transfer speed for PC/XT class machines by using shifted port mapping, so enabling 16-bit instructions. This improves the performance to about 170KB/s at 4.77MHz.



  • 8-bit ISA card with standard USB socket for connection of USB flash drive
  • Line driver for external activity LEDs (12mA power budget)
  • IBM Personal Computer XT System Board Slot 8 compatibility
  • Low-cost flash memory chip for boot ROM, with 32KB usable
  • Port-mapped IO; supports both 8- and 16-bit instructions (via partial address decoding)
  • Resource requirements:
    • IO ports 2C0-2C8h
    • ROM: 32KB at C800h or D800h


Use of this design is provided subject to the lo-tech.co.uk Terms and Conditions.

Bill of Materials

Part Device Package Qty Farnell Mouser
IC1, IC3 74HCT688N DIP-20 2 382504 771-74HCT688N
IC2 SST39SF010A-70-4C-PHE DIP-32 1 1896595 804-39SF010A7CPHE
IC4 CH375 SOIC-28 1 - -
IC5 74HCT245N DIP-20 1 9591931 595-SN74HCT245N
C1-3, C5 0.1uF Ceramic Capacitor C025-030X050 4 2112751 581-SR205E104MAR or
C6 10uF Electrolytic Capacitor E2-5 1 8767084 647-UVR1C100MDD
C7 CAPACITOR, 0.1UF, 50V 0805 2 1612208 VJ0805V104MXBPW1BC
C9 0.01uF Ceramic 0603 1 - 963-UMK107B7103KAHT
C10 0.1uF Ceramic 0603 2 - 963-UMK107BJ104KAHT
C12-13 15-30pF Ceramic 0603 2 581-06035C220JAT2A
C14, C15 100uF Electrolytic Capacitor E2-5 1 - 647-UVR1A101MDD
IC Socket (DIP32) - DIL32 1 1654375 571-1-2199300-2
IC Socket (DIP20) - DIL20 3 1101349 571-1-2199298-6
JP1, JP2, JP3, LED - 1x2 Pin Header 4 1593411 855-M20-9990246
JP4 - 2x2 Pin Header 1 - 517-9612046804AR
- - 2-pin jumper 4 2396301 806-SX1100-A
U1 SN74LS33D SOIC-14 1 1752958 595-SN74LS33D
R1, R2, R4 RESISTOR, 10K, 125MW 0805 3 1612522 71-CRCW0805J-10K-E3
R3 RESISTOR, 0.125W 1% 270R 0805 1 1652970 71-CRCW0805270RJNEA
R5 RESISTOR, 0.125W 1% 2R 0805 1 1652970 71-CRCW08052R00FKEA
R6 RESISTOR, 10K, 125MW 0204/7 1 9342419 270-10K-RC
X1 HC49 Crystal, 12MHz ±0.4‰ - 1 - 774-ATS12A-E
HD1 USB Connectors RA USB A - 1 - 523-UE27AC5410H
  • Board design permits the use of a range of DIP-32 flash chips (1, 2 or 4Mb):
    • SST39SF0x0A (SST39SF010A, SST39SF020A, SST39SF040A)
    • AMIC A29010
    • AMIC chips appear to have been discontinued (as as Nov-13)
  • Regardless of the chip used, 32KB is presented at C800h (JP2 closed) or D800h (JP2 open and R4 fitted)

Note that when assembling SMT components, additional flux will also be needed (such as Edsyn FL22, Farnell order code 3059091).


  • JP1: ROM enable (closed = enabled)
  • JP2: ROM Address Selection - closed = C800h, open (and R4 fitted) = D800h
  • JP3: Slot-8/ZWS Function (closed = enabled) - requires SMT components
  • JP4: Port mode
    • Std - 8-bit transfers, registers mapped at 2C0h, 2C1h
    • Fast - 16-bit transfers, registers mapped at 2C0h, 2C2h (not supported by alpha BIOS)

An external LED can be attached to the 2-pin LED header only if the optional components (on the rear of the PCB) are populated. Current limit for LED is 12mA.

The IO port address (2C0h - 2C8h) is fixed.

Build & Test

For additional help, please send an email.


  • Find a suitable work surface with plenty of light. A car-cleaning sponge is useful to rest the PCB on since components are held nicely in place under the PCB by the soft surface, which can also be easily turned around as a whole with components not yet soldered
  • Start with the SMT components, then proceed with the least tall components, so that they are held full in place with the solder side up
  • When soldering IC sockets or ICs, ensure the notches are at the correct end as indicated by the PCB silkscreen
  • Note the correct orientation of the polarised capacitors C6, C14 and C15.
  • Wash all flux residues off once complete using isopropynol (for example with a toothbrush)
  • Check closely with a magnifying glass all joints. Check especially for bridges between SMT pins

BIOS Flashing (Programming)

The following assumes JP2 is closed, providing the ROM at C800h

The flash chip is programmed with the Lo-tech XT-CF flash utility. To program the board, make a DOS boot disk (utilities have been tested on MS-DOS 2.11, 3.3 and 6.22) and add the BIOS binary file and the flash utility. Install the Lo-tech ISA CompactFlash Adapter in the machine and boot from the floppy, then flash the ROM thus:

A:\>flash v40-bios.bin c800 


The silkscreen incorrectly documents the fixed IO port range as 260-26Fh; the CH375 is at 2C0h.


Config.sys driver (for non-bootable use) coming soon.

See Also