4 layers184.9 × 243.8 mm
Project NuXT
monotech/NuXT
MicroATX Turbo XT Motherboard, with VGA, Floppy, CF, Serial
223 parts121GPL-3.0
2 layers106 × 72.8 mm
XTIDE ROM Card
hoffman373/XTIDE-ROM-CardAn 8-bit ISA card that holds a 27C256 EPROM as an option ROM in the upper memory area of a PC/XT-class machine, typically to load the XTIDE Universal BIOS.
- Size
- 106 × 72.8 mm
- Copper layers
- 2
- Thickness
- 1.6 mm
- Parts
- 9
- Nets
- 64
- Pads
- 129
- Tracks, vias
- 294, 39
- Pours
- 2
- Routed
- Yes
- KiCad
- KiCad 10.0 (file 20260206)
- Created
- 7 Oct 2026
- Updated
- 8 Oct 2026
Overview
XTIDE ROM Card is an open-source printed circuit board by hoffman373, from the hoffman373/XTIDE-ROM-Card repository on GitHub. It is a 2-layer board of 106 × 72.8 mm with 9 parts on 64 nets, routed with 294 tracks and 39 vias.
It is built around the 27C256-17/J. Its parts include 2 capacitors, 2 ICs and 2 connectors.
Main components
1README
README.mdXTIDE-ROM-Card
An 8-bit ISA card that holds a 27C256 EPROM as an option ROM in the upper memory area of a PC/XT-class machine, typically to load the XTIDE Universal BIOS.
The schematic is a redraw of Dan Hoover's "The $4 XTIDE" concept.
Overview
- ROM: 27C256 (32 KB) EPROM in a 28-pin DIP socket
- Address decode: 74LS688 8-bit comparator in a 20-pin DIP socket
- Address select: 2x4 pin header (J1) with a SIP-5 pull-up resistor network (RN1)
- Bus connector: 8-bit ISA edge connector (
Bus_ISA_8bit) - Decoupling: 100 nF disc capacitors on each IC
How it works
The 74LS688 compares A19-A15 against the levels set on J1. It also requires AEN low (no DMA cycle) and /MEMR low (memory read). When everything matches, it pulls ROM_SEL low and the EPROM drives D0-D7 for the read.
A19 is tied to match 1, so the 32 KB window always falls between 80000h and FFFFFh. RN1 pulls SEL_A15 to SEL_A18 high. Installing a jumper pulls that bit low.
Address selection
| J1 pins | Address bit |
|---|---|
| 1-2 | A15 |
| 3-4 | A16 |
| 5-6 | A17 |
| 7-8 | A18 |
Jumper in = 0, jumper out = 1.
| ROM base | A18 | A17 | A16 | A15 | Jumpers installed |
|---|---|---|---|---|---|
| C8000h | 1 | 0 | 0 | 1 | 3-4, 5-6 |
| D0000h | 1 | 0 | 1 | 0 | 1-2, 5-6 |
| D8000h | 1 | 0 | 1 | 1 | 5-6 |
| E0000h | 1 | 1 | 0 | 0 | 1-2, 3-4 |
| E8000h | 1 | 1 | 0 | 1 | 3-4 |
- Keep the jumper off 7-8. A18 = 0 maps the ROM into A0000h-BFFFFh, which is video memory.
- C0000h is normally taken by the video BIOS.
- Pick a window that doesn't overlap video memory, a hard disk controller BIOS, or any other option ROM in the machine.
Opening the project
- Install KiCad 10 or newer. The files use the KiCad 10 format and will not open in older versions.
- Clone the repository:
text git clone https://github.com/hoffman373/XTIDE-ROM-Card.git - Open
xtide_rom_card.kicad_pro.
Custom footprints live in lib/footprints/ and are registered through the project's fp-lib-table using ${KIPRJMOD}, so no library setup is needed. Everything else uses stock KiCad symbols and footprints.
Programming the ROM
Burn the ROM image (for example, an XTIDE Universal BIOS build configured for your controller) to a 27C256 with any EPROM programmer. If the image is smaller than 32 KB, pad it to 32 KB with FFh. The BIOS checksum must be valid or the system BIOS will skip the ROM during its option ROM scan.
License
CC BY-SA 4.0, share-alike with the original $4 XTIDE design.
Parts
9| Qty | Value | Footprint | References |
|---|---|---|---|
| 2 | 100nF | Capacitor_THT:C_Disc_D5.0mm_W2.5mm_P5.00mm | C1, C2 |
| 2 | MountingHole | MountingHole:MountingHole_3.2mm_M3 | H1, H2 |
| 1 | 27C256 | Package_DIP:DIP-28_W15.24mm_Socket | U2 |
| 1 | 74LS688 | Package_DIP:DIP-20_W7.62mm_Socket | U1 |
| 1 | Bus_ISA_8bit | myproject:PC_AT_8BIT | J3 |
| 1 | Conn_02x04_Odd_Even | Connector_PinHeader_2.54mm:PinHeader_2x04_P2.54mm_Vertical | J1 |
| 1 | R_Network04_US | Resistor_THT:R_Array_SIP5 | RN1 |
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Questions
How big is the XTIDE ROM Card PCB?
It measures 106 × 72.8 mm and has 2 copper layers, 1.6 mm thick.
What parts does the XTIDE ROM Card use?
9 parts, built around the 27C256-17/J. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.
Where can I download the XTIDE ROM Card KiCad files?
From the hoffman373/XTIDE-ROM-Card repository on GitHub, at the commit this page was made from: the board is xtide_rom_card.kicad_pcb and its schematic xtide_rom_card.kicad_sch. PCBKit opens the board in the browser, with nothing to install.
Can I use the XTIDE ROM Card design in my own project?
Its repository states no licence, so its author keeps all rights: ask hoffman373 before reusing the design.