![](http://datasheet.mmic.net.cn/390000/SYM53C710_datasheet_16836331/SYM53C710_47.png)
SCSI Bus Interface
2-27
devices may be used (DS36954 4-channel transceiver, for instance). The
suggested value for the 15 pull-up resistors in the diagram is 680
. If
Active Negation is enabled and the chip is operating in differential mode,
the value of the pull-up resistors should be 1.5 K
. Refer to
Appendix D
for other differential interface options.
2.7.1 Terminator Networks
The terminator networks provide the biasing needed to pull signals to an
inactive voltage level, and are required for both SE and differential
applications. Terminators must be installed at the extreme ends of the
SCSI cable, and only at the ends. No system should ever have more or
less than two sets of 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
pull-up resistor to the termination power
supply (Term Power) line and a 330
pull-down resistor to ground.
Differential cables use a 330
pull-up resistor from “– SIG” to Term
Power, a 330
pull-down resistor from “+ SIG” to ground, and a 150
resistor from “– SIG” to “+ SIG”.
Because of the high-performance nature of the SYM53C710, Alternative
Two SE termination is recommended. This method employs a 2.85 V
regulator and 110
pull-up resistors (no pull-down).
Figure 2.10
and
Figure 2.11
show two examples of schematics for Alternative Two
termination. For more information on Alternative Two termination, refer to
the SCSI-2 specification.
2.7.2 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 initiator mode)
tries to select one target and is reselected by another. The analogous
situation for target devices is being selected while trying to perform a
reselection.
The SYM53C710 can handle this condition in a manner identical to the
SYM53C700, that is, autoswitching between initiator and target modes,
but the recommended method is to disable the autoswitching utility.