oldcomp:nabu:expansion_cards

NABU expansion cards

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

Expansion cards which were available from NABU:

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
08 NABU Serial port
10 NABU FD1797 floppy disk
50 NABU CompactFlash adapter v2 by Randomvariations
E8 NABU Winchester hard disk controller (Xebec)

Bold entries are official NABU expansion cards.

Non-compliant cards which have no ID byte:

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.

PCB front PCB back

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.

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  • Last modified: 2024/06/13 13:21
  • by philpem