
SYM53C7XX, SYM53C8XX to 75LBC976/76A Differential Interface
D-3
D.2 SYM53C7XX, SYM53C8XX to 75LBC976/76A Differential
Interface
Following is a suggested interface between the SYM53C700,
SYM53C700-66, SYM53C710, SYM53C720, or SYM53C820 SCSI I/O
Processors (SIOP) and the Texas Instruments 75LBC976 differential
transceiver. This appendix does not apply to the SYM53C810, which has
a Single-Ended (SE) interface only. The 75LBC976 is capable of 10
Mbytes/s transfers and contains nine differential transceivers in one
package, which reduces the board space required to implement a
differential SCSI interface. The purpose of this appendix is to guide
people in their own designs. The designer should perform a thorough
design analysis before implementing this interface.
D.2.1 Differential Mode
The SIOP must be placed in differential mode by setting the DIF bit, bit
5 of the STEST2 register (0x4E) in the SYM53C720 and SYM53C820,
or bit 0 of the CTEST7 register (0x1B) in the SYM53C700, SYM53C700-
66, and SYM53C710. Setting this bit 3-states the BSY/, SEL/, and RST/
pads so they can be used as pure input pins. In addition to the standard
SCSI lines, the following signals are used during differential operation by
the SIOP in all bus modes:
Signal
Function
BSYDIR, SELDIR, RSTDIR
Active high signals used to enable 75LBC976 differential drivers as
outputs for SCSI signals BSY/, SEL/, and RST/ respectively
SDIR[15:0], SDIRP[1:0]
Active high signals used to control direction of 75LBC976 differential
drivers for SCSI data and parity lines, respectively
IGS
Active high signal used to control direction of 75LBC976 differential
driver for initiator group signals ATN/ and ACK/
TGS
Active high signal used to control direction of 75LBC976 differential
drivers for target group signals MSG/, C/D/, I/O/, and REQ/
DIFFSENS
Input to SIOP used to detect the presence of a SE device on a
differential system. If a logical zero is detected on this pin, then it is
assumed that a SE device is on the bus and all SCSI outputs will be
3-stated to avoid damage to the transceiver