ISA ROM Board Updated

The Lo-tech ISA ROM Board is a simple and cheap to make 8-bit ISA board that provides a 32 or 64KB ROM via a flash chip, and was first released in 2012 primarily to help with XT-IDE Universal BIOS development. It now enters it’s third revision, adding PC/XT Slot-8 support.


The board retains all the previous features, including selectable 32KB or 64KB operating modes, in-system programming, extensive address selection options, and entirely through-hole component choice for easy home assembly.

This kit is ideal for anyone tinkering with IBM PC BIOS or option ROM development, or as a handy flash ROM to add option ROMs for example to support high density floppy, IDE controllers without their own ROM, or network boot (like ATAoE).


CADSoft Eagle PCB design files and associated Gerbers (which are used by PCB fabricators) are all available now (with the usual Lo-tech GPL derived license) via the site wiki.

PCBs and full kits will be available via the Lo-tech shop at the start of May.

1MB RAM and 2MB EMS Boards Updated

Introduced back in February, the Lo-tech 1MB RAM Board and 2MB EMS Boards have been undergoing basic testing since, and have now both been updated to rev.2 designs, with a number of fixes and changes to make them ready for release. The designs files for both boards are available now via the site wiki.

1MB-RAM-Board-r02-Top 2MB-EMS-Board-r02-front

Along with a few component changes and the addition of configuration information on the silkscreen, the main changes are the addition of PC/XT slot-8 compatibility to the RAM Board, and a better choice of IO port addresses on the EMS Board.

Both boards now also use the same mounting hole placement, meaning a new ISA slot bracket, which is currently in development.


CADSoft Eagle PCB design files and associated Gerbers (which are used by PCB fabricators) are all available now (with the usual Lo-tech GPL derived license) via the site wiki. The EMS driver source will be added soon.

PCBs and full kits will be available via the Lo-tech shop at the start of May.

New Lo-tech PCBs: RAM & EMS

RAM chips in early 1980′s PCs are a fairly regular cause of problems, and then there’s the issue of only have some meager amount of RAM installed on the system board, as little as 16KB on the first IBM 5150.

Mostly RAM is expanded up to the maximum (usually 640KB) via a multi-function ISA expansion card, but these boards don’t provide upper memory blocks (above 640KB) nor generally EMS, a memory expansion technology for 8088/8086 PCs providing up to 32MB defined by Lotus, Intel and Microsoft. For that, something like an Intel AboveBoard is required, which is a full-length and now rare card.

So, enter two new lo-tech PCBs, both built on AS6C4008 4Mb SRAM chips:


1MB RAM Board

The lo-tech 1MB RAM board, providing from 48KB to 1MB of system RAM, with each 64KB page individually switchable to provide a universal expansion board for any 8-bit PC, regardless of how much RAM is installed on the system board. The first 16KB can also be switched off, enabling its use with a stock 16KB 5150.

2MB EMS Board

2MB EMS Board

The lo-tech 2MB EMS board, providing from 512KB to 2MB of LIM 3.2 expanded (EMS) memory (available capacity is dependent on how many SRAM chips are populated). Applications like Lotus 1-2-3 and Windows 2.x and 3.0 will use EMS when available.

Both boards are built on 1.27mm pitch SMT components in order to fit everything on the available 80x100mm Eagle Lite routing area. Assembly of these components is perfectly acheivable at home – see the lo-tech SMT soldering guide.

These boards are both in first-prototype testing phase and so should not yet be considered fully functional; some refinements are likely in future revisions. Initial test results have though been positive – using the lo-tech test-bench IBM 5155:

  • The RAM board is detected as configured on the DIP switches and the machine is able to run through Trixter’s 8088 Corruption (which utilises all available RAM) without issue
  • The EMS board is correctly identified by a low-level test routine and passes basic page register and fill operation tests

But, more test hours are needed and the driver for the EMS board is yet to be written – that’s work-in-progress!

ISA CompactFlash Parts Kit Available Now!

The Lo-tech PCBs are proving popular, especially the ISA CompactFlash PCB, the XT-CF-lite PCB and the TRS-80 IDE Adapter PCB.

The most common requests are for fully assembled boards or complete parts kits, so I’m now pleased to announce the availability of full kits for the ISA CompactFlash board, available now!

The kit contains everything needed to build a fully functioning board to allow the connection of a CompactFlash card, via a cheap Compactflash-to-IDE converter like this one or this one (randomly picked; I have no connection with the sellers) to an 8088 or 8086 based IBM compatible PC.

In the parts bag are the ICs and sockets pressed into some antistatic foam, and a number of loose components – everything on the Bill Of Materials:

ISA CompactFlash Parts Kit Contents

ISA CompactFlash Parts Kit Contents

The loose components are:


Once assembled, the XTIDE Universal BIOS must be programmed onto the card. This can be performed with the card installed in the PC (no external EPROM programmer is required) using the lo-tech Flash utlity and the pre-configured ROM image. This requires having some way to transfer the utility and image file to the target system, for example by floppy disk. If you don’t have this capability, the ROM can be programmed in an external programmer, or the kit can be shipped with the ROM ready-programmed – just add the Flash Chip Programming Service for each kit purchased.

TRS-80 IDE Hard Drive Interface


A thread on forum recently caught my eye, member Firebox have breadboarded a simple and low-cost IDE adapter based on a design by Larry Campanell for the TRS-80 Model 4 range, enabling connection of standard IDE drives.  It seemed like an ideal project to turn into a lo-tech PCB.

It turns out there’s already a bootable IDE solution, the FreHD project, providing access to disk images stored on a FAT32 formatted SD card powered by a PIC microcontroller. It does look fantastic, but I felt there could be space for something simpler (and so cheaper) as well.

The prototyped low-cost adapter needs just a few 7400 series ICs, and simply ignores half the data coming from the IDE device to provide the 256-byte sectors the TRS-80 expects. The designer though notes some compatibility issues:

I did have one problem, though — not all IDE drives would work….only one of the four IDE drives I tried would work.

That sounded familiar – I’d been through the same developing the CPLD logic for the XT-CF series.


When the CPU accesses a device, it first provides the address (IO Port) then the read or write signal.  In the Z80 world the CPU generates the same address, read (RD) and write (WR) signals for memory or IO port addressing, the two being distinguished with IO-Request (IORQ) or Memory Request (MREQ) signals.

In the TRS-80 expansion interface (model 3/4/4P), the system combines IORQ, RD and WR signals to provide simple IN and OUT signals along with the address bus, also providing IORQ and M1 signals for reference which can be used to identify interrupt acknowledgement.

In the original design it seems that device compatibility issues might have been caused by the inclusion of IORQ in the address matching logic:


IORQ is asserted concurrently with IN or OUT, so the logic has a timing issue since the IDE interface expects its chip-select (i.e. address match) line to have been asserted before the read or write command.

The Lo-tech design follows more the the XT-CF design, with the IDE chip-select line being driven directly from the address bus with just an LS688 comparator. Some prototyping work by vcforum member Chromedome45 soon proved the logic – so the Lo-tech TRS-80 IDE Adapter PCB design:


Lo-tech TRS-80 IDE Adapter (image generated with GerbV)

Further information on the board including bill of materials, device drivers and design files can be found on the Lo-tech TRS-80 IDE Adapter wiki page.


PCBs are available now through the lo-tech shop page.

Shugart and the Bubble


Magnetic Bubble Memory (MBM) was positioned as a major contender for computer storage in the late 1970s, even appearing in popular culture when at the center of the plot in Knight of the Phoenix.  The technology seemed to offer everything flash offers today, and is the basis of some surprising patents such as this iPod forerunner filed in 1979.

The TMS 9916 Bubble Memory Controller datasheet surmised,

…the main advantages are the low entry price versus disks…, nonvolatility…, and high-density storage… ideally suited for portable applications as well as providing memory for traditional processing systems

What Is Bubble Memory?


Bubble memory works like a shift register

MBM works by storing bits as cylindrical magnetic domains (the bubbles) on a film mounted within an electromagnet array, which is used to move the bubbles through the film.  Functionally, MBM is like a massive shift register – moving the bubbles results in one (stored) bit being read from one end, and a new bit being supplied at the other.

By the end of the 1970s, practically every major semiconductor manufacturer was working furiously on this technology, and of course Intel was no exception with its subsidiary ‘Intel Magnetics’.  Their 1979 Design Handbook for the Intel Magnetics One Megabit Bubble Memory is well worth a look, and includes a technical article as an appendix with some performance specifications.  The power consumption jumps out as surprisingly high for a solid-state technology.

Conner & Shugart


ST-506 (image:

Meanwhile Finis Conner and Al Shugart, having enjoyed some success as Shugart Associates with 8″ floppy and fixed disks, in 1979 formed a startup to produce 5.25″ fixed disks for the emerging personal computer industry.  Shugart Technology was born, named deliberately in the expectation of some free publicity from Shugart Associates owner Xerox.

Less than a year after incorporation, they led the storage industry with the introduction of the micro-Winchester ST-506, the first 5.25″ form factor hard drive.  With 5 MB usable capacity, it immediately provided a problem for bubble technology – and DEC (absorbed by Compaq in 1998) provided Shugart with their first major order, ST-506 drives being destined for its Rainbow PC in a deal apparently negotiated over cocktails and written on a napkin (source).

Disk vs Bubble


Intel IMB-100 Development Board

The limitations of MBM were, by 1980, becoming clear.  Whilst requiring complex controller logic similar to that of a hard disk, it was power hungry and slow in comparison: Intel’s iSBC254 bubble memory storage board needed 32W per ½MB, had a 48ms access time, burst data rate of 48KB/s and a physical volume of

In contrast, the ST-506 drive boasted 10x the capacity in a package about 3x the volume, had 10x the burst data rate, and needed less than a tenth of the power per KB.  The death-knell for bubble though came when, setting the pace of capacity expansion we’ve expected since, what was by then Seagate produced the 10MB ST-412 in 1981.

Originally to be named ST-512, the drive heads were changed to thin-film type and name updated to ST-412 according to a comment on theregister, but either way the significance in vintage computing is simple: it was selected by IBM for it’s PC/XT 5160 (produced from 1983 to 1987), putting Seagate on track to become one of the most successful storage vendors today.

Meanwhile with bubble further compounded by poor yields and the need for gadolinium gallium garnet and highly toxic chemicals, by the early 1980s pretty much all efforts with bubble memory had been dropped.

ST-506/ST-412 Reliability

Even as of 2013, there’s no shortage of working ST-412 drives – I acquired one recently in an IBM PC/XT, which when powered on presumably for the first time in about 20 years booted up straight-off, loading the school accounts system it still stored without a fuss.

Running a low-level format (using the IBM XT Diagnostics disk – the formatter is hidden away in the diagnostics menu under fixed disk tests) gives a good indication of the state of a drive, and it completed without even the hint of a bad sector.  Not bad for a 28-year old drive, especially one whose “magnetic disks have a life expectancy of 5 years” according to the service manual.


The ST506 interface used by the ST-506 and ST-412 drives was derived from the SA1000 MFM floppy interface Shugart designed in the mid 1970s, and in the PC/XT it provided 5Mbps and 17 sectors per track.  Raw transfer rate is therefore about 600KB/s, but in practice much lower because an interleave was needed to allow the CPU time to collect sector data.  Running my simple disk test utility the disk does about 68KB/s with the default 6:1 interleave.

Bubble Today

Bubble memory has popped up again more recently – MIT proposed a microfluidic bubble memory in 2007 (ultimately too slow) and IBM has been working on racetrack memory for some time.  Whilst (as of 2013) it seems unlikely that bubble will be back any time soon, IBM Research suggests that Storage Class Memory (i.e. racetrack) could become mainstream if work on 3D NAND Flash fails to deliver.

Sinclair PC200 XT-CF Card


A few weeks back I wrote about the lo-tech ISA CompactFlash Adapter designed to fit the Sinclair PC200, at the request of a system owner, and based on a few ideas I already had on the drawing board at the time.

Being simple to make and cheap, the adapter has found a home in many machines besides the Sinclair already – only a couple of PCBs are left and the feedback from assemblers has been good.

sinclair-pc200-with-lo-tech-isa-compactflash-adapter-fittedThe main challenge with the Sinclair is the available expansion slot height, which is what the small form-factor adapter was designed to solve.  Here it is fitted to the Sinclair, with the top cover fitted.

Since the Sinclair doesn’t have any spare power connectors, make use of the 4-hole power outlet on the PCB to attach a floppy-drive style power lead to power an IDE to CompactFlash adapter (alternatively the keypin on the adapter will supply 5V, if the CompactFlash adapter in use supports this).

xtidecfg-ide-controllers-settingTesting, after a couple of false starts, has been successful so far.  Be sure to set ‘IDE Controllers’ to 1 (and the adapter type to XT-CF of course) when configuring the XTIDE Universal BIOS.  The BIOS can be written out using the lo-tech XT-CF flash utility.

XT-CF Card for Tandy 1400 Series Laptops


Tandy helpfully included an expansion slot in their 1400 series of laptops, and in places there is reference to an expansion box, but it seems it never made it to market.  The later 1400FD and 1400HD models retained the expansion slot and added a second (slightly different) slot for an MFM HDD controller, as implemented in the 1400HD.

The expansion slot is basically an 8-bit ISA slot, but with a different pinout and a few differences, in a custom card form factor to fit in the machine.  Power budget is also limited to 200mA, according to the service manual.  Fortunately, Tandy documentation provides everything needed to create a card – so here is what I believe to be the first ever expansion card for Tandy 1400 Series laptops (only about 20 years late):


Expansion Card Design

Based on the information available in the Tandy technical reference, I’ve created a wiki page detailing everything about the expansion slots.  Some of the Tandy documentation is contradictory, but my wiki is based on what is now a proven design.  I’ve also included an Eagle layout for the PCB (restricted to a 100 x 100mm footprint, to enable low-cost manufacture by SeeedStudio).


My XT-CF cards provide hard disk functionality to PC/XT and PC/AT class machines based on CompactFlash (or microdrives), and for the Tandy 1400 the design needs just four ICs – a flash ROM, two 74688 address decoders and a 74139 decoder.  Being XT-CF compatible, the design is fully supported by the XTIDE Universal BIOS (from build r554).

BIOS Initialisation

I built this board a while back, and although BIOS flashing went OK the machine didn’t want to initialise the XTIDE Universal BIOS.  The BIOS was clearly detecting the option ROM as the floppy seek test was performed on only the first floppy with it present (the BIOS assuming that an HDD would be installed in place of the second floppy, exactly as the 1400HD was shipped), but the BIOS initialisation messages never appeared.

This has had me stumped and the board simply sat on the side since.  But recently XTIDE Universal BIOS project lead Tomi posted a code update (in r552):

XTIDE Universal BIOS can now be initialized if non-standard main BIOS does not call INT 19h or if INT 19h handler is replaced by some other BIOS.

And sure enough, the BIOS fired into life and the machine booted (and yes, the SuperTwist LCD screen really does look this bad):


The solution isn’t quite perfect – the fixed disk is inaccessible when restarted via CTRL-ALT-DELETE, but since boot time on this machine is identical for both soft boot and cold boot, this is just something that we need to live with for now.

1400LT, 1400FD and 1400HD

For 1400FD systems at least with BIOS 1.04, the system BIOS assumes the second floppy isn’t installed when the XT-CF option ROM is present (this may also affect 1400LT systems).  For now this is a limitation, but with 32KB of flash ROM available it should be possible to resolve it by adding a floppy BIOS to the card.

For 1400HD machines, the MFM controller must be removed since both cards have their BIOS at C800h (upper memory space is somewhat limited in the 1400 series as Tandy included RAM in the upper memory area for use as a RAM disk).


Using the ‘XTplus’ XTIDE Universal BIOS build (thanks to the V20 CPU), DOS throughput (as measured with my own test utility) is at least 550KB/s.  Due to the 8-bit data bus and V20 microcode optimisations, there is no performance difference between standard 8-bit PIO and BIU offload modes (as set with XTCFMODE), although both modes are supported.


ENIG PCBs (gold plated) are available now through the shop page.

Components will also be needed from your local electronics outlet such as Farnell, Mouser or Digikey – full Bill of Materials in the wiki.  There is no bracket needed, since the card slides into the expansion slot guides within the system chassis, and the fit into the slot is tight enough not to need and further support.

8237 DMA Transfers Across Page Boundaries


The 8237 DMA controller in the original PC/XT (and its clones) is fundamentally an 8-bit device with a 16-bit address space – perfectly matched to the MCS 85 family of which it was a part.  So to make it work with the 20-bit address space of the 8086 and 8088, IBM added a 4-bit ‘page register’ for each of its four DMA channels using a 74LS670 (a quad 4-bit register file).

The 8237 and the 74LS670 though are broadly independent; the page register does not automatically increment when the address register wraps around to zero.  This has two implications: normal segment:offset addresses must be converted to a linear, 20-bit physical address, and DMA transfers cannot cross a 64 KB page boundary.

Determining the Physical Buffer Address


Code in the XTIDE Universal BIOS illustrates how to convert a standard segment:offset address (presented in ES:SI) to a linear address, with just the 8088 instruction set:

     xor        dx, dx      ; clear DX
     mov        ax, es      ; copy ES to AX
 %rep 4 
     shl        ax, 1       ; shift left 1, MSB into carry...
     rcl        dx, 1       ; ...and from carry to DX LSB
 %endrep                    ; repeat for the 4 MSB bits
                            ; AX now has ES SHL 4, and DX has ES SHR 12 
     add        si, ax      ; add AX to SI, to get low 16-bits in SI
     adc        dl, dh      ; if it overflowed, increment DX (DH is zero)
     mov        es, dx      ; and save DX back in ES

DX needs to end up with ES SHR 12 because IBM hooked up the 74LS670 DMA page register to the low four-bits of the data bus, so programming the high 4-bits of the physical address is achieved from the low 4-bits of a CPU register.  The addresses are then loaded into the DMA controller address register (in two halves, since the DMA controller has only an 8-bit data bus) and the associated page register.  In this example, the port addresses are for channel 3:

     out        0Ch, al                ; Reset flip-flop to low byte 
     mov        ax, es                 ; Get high 4 bits
     out        82h, al                ; Page register for Ch.3 
     mov        ax, si                 ; Get low 16 bits
     out        06h, al                ; Send low byte to Ch.3 address register
     mov        al, ah                 ; 
     out        06h, al                ; Send high byte to Ch.3 address register

Crossing a 64KB Boundary

Since the page register isn’t incremented by the DMA controller, a DMA transfer can run up to a page boundary at which point it (and the associated page register) must be re-programmed for another transfer into the next physical page.  Splitting a transfer across a boundary therefore requires a check of the transfer size against the possible number of bytes up to a page boundary.

The code that follows assumes the maximum total transfer size is less than 64KB so allows for either one or two DMA transfers.

    ; On entry - buffer is in ES:DI, CX has bytes to transfer
    ; First calculate bytes up to physical page boundary
    mov        ax, di 
    neg        ax                 ; 2s compliment

    ; if DI was zero, carry flag will be cleared (and set otherwise)
    ; When DI is zero only one transfer is required if total DMA
    ; transfer size is restricted to < 64KB
    jnc    .TransferDmaPageWithSizeInCX

    ; CF was set, so DI != 0 and we might need one or two transfers
    cmp        cx, ax                    ; if won't cross physical page boundary... 
    jbe    .TransferDmaPageWithSizeInCX  ; ...perform transfer in one operation 

    ; Calculate how much we can transfer on first and second rounds 
    xchg        cx, ax            ; CX = BYTEs for first page 
    sub         ax, cx            ; AX = BYTEs for second page 
    push        ax                ; Save bytes for second transfer on stack 

    ; Transfer first DMA page 
    call    StartDMAtransfer 
    pop         cx                ; Pop size for second DMA page 

    ; Fall to StartDMAtransfer 

    ; DMA controller programming and transfer is completed here
    ; This code will be hardware dependent
    ; ...

    ; Once transfer is done, update physical address in ES:DI
    ; since IO might need several calls through this function
    ; (if crossing a physical page boundary)
    mov        ax, es             ; copy physical page address to ax 
    add        di, cx             ; add requested bytes to di 
    adc        al, 0              ; increment physical page address, if required 
    mov        es, ax             ; and save it back in es 



The DMA controller in the original IBM PC really has a few reasons for being – RAM refresh of course, background transfers (as used by SoundBlaster sampled audio for example), and high-performance transfers.  The 8237 DMA controller is usually noted for its lack of performance, but that perception came about because CPU speed soon eclipsed it.

Operating in a 4.77MHz 8088, the DMA controller is the only way to transfer data to or from a peripheral in consecutive, back-to-back full-speed bus cycles (in later PCs, the DMA controller is throttled to about 5MHz to ensure peripheral compatibility).  Of course it’s made more difficult by the boundary crossing issues and requirement to pause for RAM refresh, but the controller can provide the fastest possible transfers as demonstrated by my XT-CFv3 DMA Transfer Mode.

Lo-tech ISA CompactFlash Board

Another PCB :)

Lo-tech ISA CompactFlash Adapter

I was contacted recently by the owner of a Sinclair PC200 (an XT class machine in an Atari ST style case) about my XT-CF boards, wondering if a custom board could be built for the machine, with its physical 50mm height restriction on its two ISA slots.  The quick answer to that is no, since the CompactFlash header is too big in either orientation, but I’d already been pondering on the idea of a super-simple PCB providing a 40-pin IDE header, based on the lo-tech 8-bit ROM board, so this seemed like a good opportunity to finish the design.

Lo-tech ISA CompactFlash Board: What is it?

A through-hole, small form factor 8-bit ISA adapter providing a 40-pin IDE header suitable for connection to a separate IDE to CompactFlash adapter (available cheaply on eBay).  By using a 40-pin header instead of a CompactFlash socket, home assembly of the device is made simple as all required components are 2.54mm pitch through-hole.

Other than the small form factor (and without any provision for a slot bracket), the PCB is a standard ISA card and can be used in any PC with an ISA slot.

New IDE Logic Implementation

Making an XT/IDE derivative in such a small PCB meant minimising the component count:

  • Whilst the XT-CF-lite was based closely on the original XT/IDE adapter, borrowing directly its OR and inverter gate design to provide IDE register access and IDE reset, with this new board this has been condensed into a single 74LS139.
  • Because of the 40-pin potentially cabled connection to the CompactFlash adapter, a buffer (74HCT245) is required, but with only 8-bit transfer mode supported (hence the CompactFlash designation), a 16-to-8-bit MUX isn’t required

Any CompactFlash or microdrive device should work with this adapter, and probably ATA-2 compliant hard drives (since 8-bit transfer mode is a requirement of ATA-2).

Extended Functionality

I wanted to include as much functionality as possible, so implementing the logic for IBM PC/XT Slot-8 compatibility (developed with the CPLD based XT-CF adapters) was a goal.  Although out of space on the ISA component side, the layout could accommodate a few SMT components on the ISA solder side: these are entirely optional, but when populated provide LED drive and Slot-8 functions.

The logic required to provide slot-8 functionality is straight-forward – it’s just an echo of MEMR or IOR when either the ROM or IDE ports (respectively) are selected, with an open-collector drive since other logic on the system board can also drive this signal (as I’ve written about before).  In this new board, this is implemented with a single 74LS33D quad NOR gate, the one spare gate being used to provide LED drive too:

Lo-tech ISA CompactFlash Adapter SMT Logic

LED drive requires the SMT 74LS33D populated since CompactFlash devices provide only minimal activity LED drive current, which is likely already absorbed by LEDs on the required CompactFlash to IDE adapter.

The boards are fully supported by the XTIDE Universal BIOS (from build R545), with standard 8-bit PIO transfer mode and the enhanced performance BIU Offload mode just as with the XT-CF-lite.  The DMA transfer mode of the XT-CFv3 is not however supported.


ENIG (gold plated) rev.2 PCBs are available via the shop page.

Components will also be needed from your local electronics outlet such as Farnell, Mouser or Digikey – full Bill of Materials in the wiki.