NABU expansion cards

The NABU has four internal expansion slots, which allow extra hardware to be added.

Expansion cards which were available from NABU:

Floppy disc controller (WD1797)
Hard disc controller (connects an external Western Digital 1001 controller to the NABU bus)
- The WD1001 bus is the 8-bit precursor to IDE/ATA, sometimes called XT-IDE or XTA (XT Attachment).
Serial port

A standard NABU PC needs to have an Option Card fitted to allow expansion cards to be fitted. The Option Card converts the four independent NABU expansion ports into a single connector which daisy-chains from card to card. This makes the inside of the machine tidier, and reduces the total cable length and bus loading.

The standard NABU PC expansion ports (motherboard J9 through J12) are wired as follows:

Pin	Name	Function
1	`JxINT`	interrupt request (for this slot)
2	`/OM1`	M1 output from Z80 CPU. Indicates instruction fetch and interrupt acknowledge.
3	`O3.58`	3.58MHz clock
4	`JxAIN`	audio input from expansion card (for this slot)
5	`/CSx`	chip select (for this slot)
6	`OA0`	address bus LSB
7	`OA1`	address bus bit 1
8	`OA2`	address bus bit 2
9	`OA3`	address bus bit 3
10	`O1.79`	1.79MHz clock
11	`/OWR`	bus write cycle (from Z80)
12	`/ORD`	bus read cycle (from Z80)
13	`/OIORQ`	bus I/O request (from Z80)
14	`/WAIT`	wait state request (to Z80)
15	`/RST`	negative-going reset
16	`OD0`	data bus LSB
17	`OD1`	data bus bit 1
18	`OD2`	data bus bit 1
19	`OD3`	data bus bit 1
20	`OD4`	data bus bit 1
21	`OD5`	data bus bit 1
22	`OD6`	data bus bit 1
23	`OD7`	data bus MSB
24	`+5V`	+5V power supply
25	`+5V`	+5V power supply
26	`GND`	Ground
27	`GND`	Ground
28	`GND`	Ground
29	`+12V`	+12V power supply
30	`-12V`	-12V (negative) power supply

Note: pins marked “(for this slot)” are specific to one connector. All pins without this designation are connected to the same-named pin on every slot.

The Option card combines the four motherboard expansion ports into a single 30-pin header which can be daisy-chained to all the I/O cards.

This improves the wiring inside (four long cables are replaced with four shorter daisy-chained cables) which in turn makes the machine easier to service, but also reduces the capacitive and inductive load the cable places on the system bus.

According to Vintagecomputer.ca, an 8K Version 2 ROM is required for the Option Card to work. This requires a small motherboard modification if a 4K (non-floppy) ROM was originally fitted.

The “option card” J9A output port (which connects to the first expansion card in the chain) is wired as follows:

Pin	Name	Function
1	`/RST`	negative-going reset
2	`/OM1`	M1 output from Z80 CPU. Indicates instruction fetch and interrupt acknowledge.
3	`O3.58`	3.58MHz clock
4	`GND`	system power supply ground
5	`J9AIN`	audio input from expansion card
6	`OA0`	address bus LSB
7	`OA1`	address bus bit 1
8	`OA2`	address bus bit 2
9	`OA3`	address bus bit 3
10	`GND`	system power supply ground
11	`/OWR`	bus write cycle (from Z80)
12	`/ORD`	bus read cycle (from Z80)
13	`/OIORQ`	bus I/O request (from Z80)
14	`/WAIT`	wait state request (to Z80)
15	`OD0`	data bus LSB
16	`OD1`	data bus bit 1
17	`OD2`	data bus bit 1
18	`OD3`	data bus bit 1
19	`OD4`	data bus bit 1
20	`OD5`	data bus bit 1
21	`OD6`	data bus bit 1
22	`OD7`	data bus MSB
23	`J9INT`	interrupt request from first card
24	`J10INT`	interrupt request from second card
25	`J11INT`	interrupt request from third card
26	`J12INT`	interrupt request from fourth card
27	`/CS0`	address decode/chip select for first card
28	`/CS1`	address decode/chip select for second card
29	`/CS2`	address decode/chip select for third card
30	`/CS3`	address decode/chip select for fourth card

Power connector J16 carries the following voltages:

Pin	Name	Function
1	`GND`	Ground
2	`GND`	Ground
3	`-12V`	-12V (negative) power supply
4	`+5V`	+5V power supply
5	`+12V`	+12V power supply

I/O cards are accessed at the following addresses:

Card no.	Connector	I/O address MSN
1	J9	`Cx` hex
2	J10	`Dx` hex
3	J11	`Ex` hex
4	J12	`Fx` hex

Per (MacGyver75, I/O cards have an ID byte in the topmost address (CF, DF, EF or FF hex). This means cards may be in any slot, and the NABU will detect them automatically.

The following ID bytes are known:

ID byte (hex)	Card type
`10`	FD1797 floppy disk
`E8`	Hard disk controller
`08`	Serial port

Non-compliant cards:

NABU-IDE doesn't implement card ID detection
NABU-SD (currently)
NABUduino

The option card is required for connecting expansion cards to a NABU PC.

It combines the four ports on the NABU motherboard into a single port, which is then connected up to the first expansion card. Subsequent expansion cards daisy-chain off the previous one.

The motherboard slots have slot-specific interrupt and chip-select pins; the option card combines these into one slot, at the cost of losing the power supply pins.

Thanks to v1ph3rpunk on the NABU PC Discord for these.

Thanks to MacGyver65 on the NABU PC Discord for this.

J9a is the pin header which connects to expansion cards.

J9 through J12 connect to the motherboard headers.

The audio outputs from cards 10 through 12 are not connected. This means only one card can output audio, unless the cards with sound output can be enabled and disabled.

There may be inaccuracies in the serial port schematic, see this discussion on the Nabu PC Discord. R6 and R7 may be incorrect.