Last updated on 10 May 2026 by Suffocation

Why take a card with just one ROM when you can get one that has two? That's exactly what the developers at Monotech probably thought, and they've laid out a Double ROM card, and even better, they've made the hardware open source. So, if you don't want to tinker yourself, you can get here… here buy the original assembled with many other beautiful projects (This advert is not sponsored 😉).

ADDENDUM With every new circuit board with switches, I usually have the problem that my switches don't fit the stock. This time I ordered SMD switches and bent the pins so that they fit into the holes.

Link to the project

Link to the project

Views

Facts

• Space for two EEPROMs.
• For each ROM, you can choose between 8K and 32K (the labelling on the circuit board is incorrect)
• The address can be set using the DIP switches next to the relevant ROM.
• Two DIP switches are reserved for the ROM setup: ROM Active and Write Permission.
• A 74LS688 under the ROM handles the connection to the bus (chip select)

Circuit diagrams

Documentation

The DIP switch and the switch to the right of the socket each control the settings for that socket. The available settings are the same for both sockets. It is also possible to use just one ROM; if two ROMs are used, care must be taken to ensure that they use different address ranges.

DIP switch

Here is a description of the DIP switch settings.

Address	1	2	3	4
C000	0	0	0	0
C200	0	0	0	1
C400	0	0	1	0
C600	0	0	1	1
C800	0	1	0	0
CA00	0	1	0	1
CC00	0	1	1	0
CE00	0	1	1	1
D000	1	0	0	0
D200	1	0	0	1
D400	1	0	1	0
D600	1	0	1	1
D800	1	1	0	0
DA00	1	1	0	1
DC00	1	1	1	0
DE00	1	1	1	1

Description	5	6
ROM Disabled	Australia	*
ROM Activated	An	*
ROM write-protected	*	Australia
ROM writable	*	An

ROM size

The small switch allows the ROM size to be set depending on the (E)PROM used.

Description	Switch position
8K for ICs 27C64, 28C64B	Above
32K for 28C256	Down

Assembly

Record

Description	Worth
Gerber available	Yes
Gerber file	Link
Ordered where	JLCPCB
Piece	10
Price	14€ including shipping and taxes in a large package
Special features	Lead-free, black

Here save the Gerber files, please on the Original page according to the current version.

Components

I ordered the components from various sources.

Component	Size	Number	Description	Price €
74LS688	Dip	2	TTL 8-Bit Comparator	0,68
Wide base	28-pin DIP	2	Base, Wide	0,2
5 x 10K	6-pin	2	Resistor array	0,2
C1, C2	100nF	2	Ceramic capacitor	0,10
South West	2*3 metres	2	Slide switch	0,4
Dip SW	12 pins	2	6 x Dip Switch	0,4
C3	10 microfarads	1	Electrolytic capacitor	0,2
			Summer	2,18€

Structure

Variations

Order

Commissioning

Connect the ROM, set the address, insert the ISA card correctly into the SLOT and hope that the ROM becomes visible on boot. For troubleshooting with XTIDE, I've already written something in the following article.

Retro – XT-IDE (BIOS)

Floppy BIOS

Source: https://github.com/skiselev/floppy_bios
Post: https://www.fambach.net/retro_pojekt-retro-projekt-floppy-disk-controller-isa-fdc/

A BIOS for operating up to 8 floppy drives from 160KB to 2.88MB.

Facts:

• Up to 8 floppy drives
• 5 1/4 and 3 1/2 inch support
• Up to two controllers
• Own BIOS functions for configuration
• 100% IBM PC XT/AT compatible
• ...

Further sources: –

XTIDE Universal BIOS

Source: https://xtideuniversalbios.org
Post: https://www.fambach.net/retro-xt-ide-bios/

Enables new and large hard drives to be operated on old hardware.

Facts:

• Up to 4 hard drives
• Up to 2 controllers
• Configuration software
• Boot menu for starting from floppy disk or other sources
• 100% IBM PC XT/AT compatible

Further sources: –

Roman

Source: https://rayer.g6.cz/romos/romose.htm
Post: https://www.fambach.net/wp-admin/post.php?post=12663&action=edit

Free-DOS on a ROM

Facts:

• DOS OS on the ROM
• For drive-less operation of PCs
• Space for your own programmes
• Detailed „Assembly Instructions“

Further sources: –

Supersoft/Landmark Diagnostic ROM

Source: Supersoft Landmark XT (No original link, leads to MinusZeroDegree)
Post: –

Diagnose BIOS to find an error. Used when an XT board is not functioning, but there are no short circuits and a good power supply. (Source: https://www.minuszerodegrees.net/)

Facts:

• No RAM error support
• XT Compatible
• CGA, MDA/MGA support
• EGA is poorly supported
• Only a few VGA cards work.
• Different BIOS versions for different XT versions
• 27256 ROM Required (e.g. 27C256)

Further sources:

User manual

Supersoft/Landmark Diagnostic ROM (AT Version)

Source: Supersoft Landmark AT Clone (No original link, leads to MinusZeroDegree)
Post: –

BIOS diagnostics for AT mainboards. Not suitable for finding memory errors.

Facts:

• No RAM error support
• AT Compatible
• Not all CGA, MDA/MGA, EGA cards are supported
• None of the tested VGA cards are working.Minus Zero Degrees)
• Various BIOS images for different ROMs
• Supports 27256 / 27512 ROMs (For example 27C256, 27C512)
• W27E257 EEProm supported

Further sources: –

Ruud's Diagnostic ROM

Source: https://www.minuszerodegrees.net/ruuds_diagnostic_rom/Ruuds%20Diagnostic%20ROM.htm
Post: –

ROMs for XT and clones

Facts:

• Versions with CGA/MDA output and without
• XT 5150, 5155, 5160, Klone
• RAM, CPU, Interrupt Test
• Keyboard Reset/Stuck Key Test
• Floppy Controller / Read Test
• Rom F4000, F6000, F8000, FA000, FC000 8-Bit Checksum Test

Further sources: –

X Ram Rest

Source: Extratest
Post:

RAM test on a ROM, good if the normal BIOS refuses to work.

Facts:

• Test of bit patterns and address/data bus
• March-U Algorithms
• No parity RAM test.
• PC/XT Clone Support (5150, 5160)
• Different ROM images for different ROM sizes
• 2764/2864/27256/28256 Support

Further sources:

IBM ROM Basic

Source: https://www.ardent-tool.com/tech/ROM_BASIC.html
Post: –

IBM Basic, which was delivered with the 5150, 5160, 5170... machines. This made it possible to start the XT PC in Basic mode. This ROM was also referred to as Cassette Basic because no diskettes were needed for it.

Facts:

• XT compatible
• Commercial product – but seems to be available without licensing
• Complete Basic interpreter

Further sources: –

https://www.pcjs.org/machines/pcx86/ibm/5150/rom/basic

https://github.com/jeffpar/pcjs/tree/gh-pages/machines/pcx86/ibm/5150/rom/basic/

https://github.com/jeffpar/pcjs/tree/gh-pages/machines/pcx86/ibm/5160/rom/basic

GLaDOS

Source: https://github.com/640-KB/GLaBIOS
Post: –

Newly written minimal BIOS for XT 8088.

Facts:

• Advanced, coloured postscreen
• Quick
• 5150 Dataset Support
• V20 instruction set support
• CGA snow removed
• ...

Further sources: –

V40 BIOS

Source: https://gitlab.com/hakfoo1/v40-bios
Post: –

This BIOS is a revision of an old BIOS and adds CH375 support. This allows with the correct hardware (for example https://www.lo-tech.co.uk/product/isa-usb-adapter/), to operate and even boot from USB sticks.

Facts:

• Does the CH375 allow controlled USB stick addressing?
• The number of floppies must be set when compiling

ROMDOS

Source: https://ftp.emacinc.com/Tech_Info/ROMDOS/ROM-DOS_Users_Guide.pdf

Facts:

• DOS 6.22 on ROM
• Commercial Product (Licensed)

Further sources:

https://archiveos.org/rom-dos

https://shop.emacinc.com/ms-dos

Super PC/Turbo XT BIOS 3.1

Source: http://www.phatcode.net/
Post: –

Facts:

• Modified version of the anonymous generic Turbo BIOS
• Compatible with IBM 5150/5155/5160 
• Also for 8/10 MHz turbo systems
• Bug fixed

Further sources:

https://github.com/virtualxt/pcxtbios/tree/master

Miscellaneous

General Tips

Soldering points

Check the solder joints on the ICs again carefully, preferably with a microscope or magnifying glass, for short circuits and contact. Measure with a multimeter or continuity tester.

For small solder joints on through-hole components, use flux if possible; it makes life easier and increases the quality of the connection.

Problems

Conclusion

Description	Worth
Successful	Yes / No
Price per piece	~3 €
Soldering time	Half an hour
Function	Gut
Is it worth it	? – Good question, apart from XT-IDE, I don't currently have an application, but it's not bad to be able to run two ROMs in parallel 😉
Restrictions	Is limited to the 3 (E)PROM types 27C64 (read-only), 28C64B, 28C256

Related Posts

• Retro Project - ISA 8-Bit Backplane
• Retro Project – Voltage Blaster
• Retro Project – 8-Bit ISA Ethernet Card (isa8_eth)
• Retro Project – FDC USB Floppy Disk Controller
• Retro Project – Floppy Disk Controller (isa-fdc)
• Retro Project – ROMOS
• Retro Project – USB to RS232 Mouse Adapter
• Retro Project - Greaseweazle 4.1
• Retro Project – ATX Form Card
• Retro Project – ISA Boot EEPROM Card

Sources

To the project

GitHub Link

Parts list

Gerber files

https://github.com/skiselev/floppy_bios

Retro pages

https://archive.org – Old software, magazines, manuals, and much more.

https://theretroweb.com – PC Hardware Directory.

https://www.winhistory.de/index.php – DOS, Windows Version Descriptions.

Win 3.1x Tools and Tricks

Retro Forensics

https://forum.classic-computing.de/forum – German/English forum with many retro computer enthusiasts.

https://www.vogons.org – I have found many tips on hardware here, they also have a driver directory.

https://www.dosforum.de – Forum with strict rules and very helpful regarding DOS, Windows questions are not welcome.

https://dosreloaded.de – DOS, hardware repairs, BIOS files and more.

https://www.winhistory-forum.net DOS and Windows history.

Retro Shops

https://ram-co-shop.de - PC parts, including newer ones, have ordered a lot from there and it's always worked.

http://www.amoretro.de – Sold on eBay and has a nice directory of old hardware

https://retroreiz.de – Atari, Nintendo, Sega and others.

https://www.retroworld.info/de – Stickers, signs and more.

https://retesa-nb.de – Older PC hardware

https://www.retro8bitshop.com – C64 tricky

Logbook

Date	Description
08.03.2025	Post Created
06.07.2025	Post finalized