
2-22
Functional Description
2.7.3 Loopback Mode
The SYM53C895 loopback mode allows testing of both initiator and
target functions and, in effect, lets the chip communicate with itself.
When the Loopback Enable bit is set in the STEST1 register, the
SYM53C895 allows control of all SCSI signals, whether the SYM53C895
is operating in initiator or target mode. For more information on this mode
of operation, refer to the
Symbios SYM53C8XX PCI-SCSI I/O
Processors Programming Guide.
2.8 Parity Options
The SYM53C895 implements a exible parity scheme that allows control
of the parity sense, allows parity checking to be turned on or off, and has
the ability to deliberately send a byte with bad parity over the SCSI bus
to test parity error recovery procedures.
Table 2.5 denes the bits that
are involved in parity control and observation.
Table 2.6 describes the
parity control function of the Enable Parity Checking and Assert SCSI
Even Parity bits in the SCSI Control Zero (SCNTL0) register.
Table 2.7describes the options available when a parity error occurs.
Table 2.5
Bits Used for Parity Control and Generation
BIt Name
Location
Description
Assert SATN/ on Parity
Errors
SCSI Control Zero
(SCNTL0), Bit 1
Causes the SYM53C895 to automatically assert
SATN/ when it detects a parity error while operating
as an initiator.
Enable Parity Checking
SCSI Control Zero
(SCNTL0), Bit 3
Enables the SYM53C895 to check for parity errors.
The SYM53C895 checks for odd parity.
Assert Even SCSI Parity SCSI Control One
(SCNTL1), Bit 2
Determines the SCSI parity sense generated by the
SYM53C895 to the SCSI bus.
Disable Halt on SATN/ or
a Parity Error (Target
Mode Only)
SCSI Control One
(SCNTL1), Bit 5
Causes the SYM53C895 not to halt operations when
a parity error is detected in target mode.
Enable Parity Error
Interrupt
SCSI Interrupt
Enable Zero
(SIEN0), Bit 0
Determines whether the SYM53C895 generates an
interrupt when it detects a SCSI parity error.
Parity Error
SCSI Interrupt
Status Zero
(SIST0), Bit 0
This status bit is set whenever the SYM53C895 has
detected a parity error on the SCSI bus.