
2-24
Functional Description
2.8.1 SCSI Termination
The terminator networks provide the biasing needed to pull signals to an
inactive voltage level. They also 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. The terminators should be socketed, so that
if not needed they can be removed, or there should be a means of
disabling them with software.
Single-ended cables can use a 220
pull-up to the terminator power
supply (Term-Power) line and a 330
pull-down to Ground. Due to the
high-performance nature of the SYM53C895, Regulated (or Active)
termination is recommended.
Figure 2.4 shows a Unitrode active
terminator. For additional information, refer to the SCSI-2 Specication.
TolerANT active negation can be used with either termination network.
For information on terminators that support LVD, refer to the SPI-2 draft
standard.
Note:
When using the SYM53C895 in a design with an 8-bit SCSI
bus, all 16 data lines still must be terminated or pulled high.
Table 2.7
SCSI Parity Errors and Interrupts
DHP1
1. Key: DHP = Disable Halt on SATN/ or Parity Error (bit 5 SCSI Control One (SCNTL1))
PAR = Parity Error (bit 0 SCSI Interrupt Enable Zero (SIEN0))
PAR
Description
0
Halts when a parity error occurs in target or initiator mode and does not
generate an interrupt.
0
1
Halts when a parity error occurs in target mode and generates an
interrupt in target or initiator mode.
1
0
Does not halt in target mode when a parity error occurs until the end of
the transfer. An interrupt is not generated.
1
Does not halt in target mode when a parity error occurs until the end of
the transfer. An interrupt is generated.