AT&T 3B2 Maintenance Reference Manual page 149

Version 2
TERMINATING
~ +---3_4_-P_l_N_C_O_N_T_R~O-L_C_A_B_L_E_(D_A_l_S_Y-_C_H_A_IN--'-)~
/ - RESISTOR
CONTROL-
I
~
SYSTEM BOARD
(DISK CONTROLLER)
DISK 0
I
DATA O
-1-----------'-
20-PIN DATA CABLE
DATA 1
20-PIN DATA CABLE
Figure 6-2. ST-506 interface
DISK 1
Care must be taken when replacing a disk drive to ensure that the ribbon cables are
reconnected properly. In the Models 300 and 310, the control cable routes from the system
board, to the internal disk, and on to the disk access panel (see Figure 6-4). The data cable
form the data 0 connector on the system board connects to the internal disk. The data cable
from the data 1 connector goes to the disk access panel. In a 3B2/400 equipped with two
disks, the control cable is daisy-chained to both disks, and data cables 0 and 1 go to disks 0
and 1, respectively.
The placement of the terminating resistor is also important. A terminator resistor must be
present in any configuration. Its purpose is to terminate the signals on the control (daisy-
chained) cable. In Models 300 and 310, the disk access panel contains two single-inline sockets,
and one single-inline terminating pack (SIP). This resistor must be in the right-hand socket,
viewed from the front of the machine, if a second disk is NOT present. If an XM disk is
connected, simply move this terminator to the left-hand slot, that is just a holder. In this
situation, the second disk must contain a terminating resistor. In the Model 400, the last disk
on the control cable must have a terminating resistor installed. See Appendix E, "Peripheral
Configuration Figures," for the location of the terminating resistor socket. These resistors are
called dual-inline packages (DIP).
The ID of the disk should be set to either 0 or 1. The ID must match the data cable. For
example, if the data 0 port on the system is cabled to the disk, its ID must also be set to 0. If
only one disk in configured, it should be set to 0.
382 Subassemblies
6-9