Lo-tech ISA USB Adapter

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.

For help getting this kit running, please see the Lo-tech ISA USB Adapter FAQ or Get In touch.

The BIOS ROM on the card enables 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.

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 board is currently powered by an experimental BIOS, however integration with the XT-IDE Universal BIOS is planned, which will yield faster throughput. 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.

Please note that this adapter is currently experimental (untested) - the first PCBs are due back from fab shortly.

To pre-order this and other PCBs, please visit the lo-tech shop.

Specifications

General

• 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

Design

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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 581-SR205C104KAR
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	571-1-2199300-2 517-4832-6000-CP
IC Socket (DIP20)	-	DIL20	3	1101349	571-1-2199298-6 649-DILB20P-223TLF
JP1, JP2, JP3, LED	-	1x2 Pin Header	4	1593411	855-M20-9990246 653-XG8S-0231
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).

Configuration

• 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.

General

This project is available in the Lo-tech store as a Bare PCB - components from the Bill of Materials above must be sourced separately for example from Farnell, Newark, Mouser, DigiKey or other electronic component supplier. Farnell and Newark part numbers are provided above for convenience. An appropriate USB flash drive must be purchased separately.

A temperature controlled soldering iron is highly recommended for assembly. Tin-lead solder is generally easier to use than lead-free. A quality syringe flux is also recommended and is essential for the optional SMT parts (see SMT Soldering Notes). Before construction, the PCB should be cleaned in isopropynol.

Construction

• 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 xt-usb.bin c800 

Errata

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

Downloads

• Alpha BIOS binary
• CH375 datasheet

See Also

• Lo-tech ISA USB Adapter FAQ
• lo-tech XT-CF Boards