Apple 250 manual Terminator

C H A P T E R 2

Hardware Modifications

Table 2-4 lists and describes the interface signals for the 8-pin segment of the connector.

Table 2-4	Hard disk SCSI connector signal assignments: SCSI ID
Pin	Signal name	Description
1, 2, 3, 4, 8	Unused	These pins are not used and are not connected
		electrically to the CPU
5	/ID1	SCSI ID 1
6	/ID2	SCSI ID 2
7	/ID4	SCSI ID 4

The internal hard disk is assigned a SCSI ID number (0–7). Pins 5-7 on the SCSI connector are encoded and allow the CPU to select the appropriate device. Table 2-5 shows how the ID signals are encoded.

Table 2-5	SCSI ID encoding
ID1: Pin 5	ID2: Pin 6	ID4: Pin 7	SCSI ID
High	High	High	0
Low	High	High	1
High	Low	High	2
Low	Low	High	3
High	High	Low	4
Low	High	Low	5
High	Low	Low	6
Low	Low	Low	7

Terminator

The hard disk has 1000-ohm termination resistors for all I/O signal lines. The lines are pulled up to through the resistors to termination power.

200 MB Hard Disk Drive 13