LSI 53C810A Scsi Bus Interface, Terminator Networks, Select/Reselect During Selection/Reselection

2.6 SCSI Bus Interface

The LSI53C810A supports SE operation only. All SCSI signals are active LOW. The LSI53C810A contains the SE output drivers and can be connected directly to the SCSI bus. Each output is isolated from the power supply to ensure that a powered-down LSI53C810A has no effect on an active SCSI bus (CMOS “voltage feed-through” phenomena). TolerANT technology provides signal filtering at the inputs of SREQ/ and SACK/ to increase immunity to signal reflections.

2.6.1 Terminator Networks

The terminator networks provide the biasing needed to pull signals to an inactive voltage level, and to match the impedance seen at the end of the cable with the characteristic impedance of the cable. Terminators must be installed at the extreme ends of the SCSI chain, and only at the ends. No system should ever have more or less than two terminators installed and active. SCSI host adapters should provide a means of accommodating terminators. There should be a means of disabling the termination.

SE cables can use a 220 Ω pull-up resistor to the terminator power supply (Term-Power) line and a 330 Ω pull-down to ground. Because of the high-performance nature of the LSI53C810A, regulated or active termination is recommended. Figure 2.3 shows a Unitrode active terminator. TolerANT active negation can be used with any ANSI approved termination network. For additional information, refer to the SCSI-2 specification.

2.6.2 Select/Reselect During Selection/Reselection

In multithreaded SCSI I/O environments, it is not uncommon to be selected or reselected while trying to perform selection/reselection. This situation may occur when a SCSI controller (operating in the initiator mode) tries to select a target and is reselected by another. The Select SCRIPTS instruction has an alternate address to which the SCRIPTS will jump when this situation occurs. The analogous situation for target devices is being selected while trying to perform a reselection.

SCSI Bus Interface 2-11