PC3086 Option links

The PC3086 motherboard has option links all over it. They are used mainly to disable builtin motherboard functions.

LK1: Drive A: Changeline

Put a jumper on this link if drive A: does not have a changeline.

LK2: Drive B: Changeline

Put a jumper on this link if drive B: does not have a changeline.

LK3: Disable power-on self test

Put a jumper on this link to skip the power-on self test.

LK4: Memory size

According to the manual, this should be left with the jumper on pins 2 and 3. The service manual implies that moving it to pins 1 and 2 reduces the memory to 512k, as on earlier XTs.

LK5: BIOS ROM size

According to the manual, this should be left disconnected. The service manual schematic suggests that, as on the PC1512, it controls BIOS ROM size. If it is connected, the ROM is treated as 32k (2x16k) rather than 16k (2x8k).

LK6: Write Precompensation

According to the manual, this should be left on pins 1 and 2 (250ns) rather than pins 2 and 3 (125ns).

LK7: Disable motherboard COM1

Put a jumper on this link to disable the motherboard COM1 port.

LK8: Disable motherboard VGA

Put a jumper on this link to disable the motherboard graphics support.

LK901, LK902: Primary display type

Both links disconnected: VGA (built-in or external).
Both links connected: MDA or Hercules (external).
Only LK901 connected: CGA 40-column (external).
Only LK902 connected: CGA 80-column (external).

LK951: IDE reset?

This is not documented in the manual; the schematic in the service manual suggests that it is to do with the IDE reset signal.

LK952: Hard drive controller address

Connect pins 1 and 2 for primary. Connect pins 2 and 3 for secondary.

LK953: Hard drive BIOS address

If no jumpers present: Disable hard drive BIOS.
If pins 2 and 3 connected: BIOS at primary address.
If one jumper on pins 1 and 2, and another on pins 3 and 4: BIOS at secondary address.

LK954: Address Enable

According to the manual, this should be left disconnected. The service manual suggests that if connected, it connects the ISA 'Address enable' signal to the hard drive.

LK955: Hard Drive IRQ

(from the service manual) If pins 1 and 2 connected: Hard drive uses IRQ5.
If pins 2 and 3 connected: Hard drive uses IRQ2.