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SYM53C876/876E Data Manual
2-17
Functional Description
SCSI Functional Description
SCSI Bus Interface
All SCSI signals are active low. The
SYM53C876 contains the single-ended out-
put drivers and can be connected directly to
the SCSI bus. Each output is isolated from the
power supply to ensure that a powered-down
SYM53C876 has no effect on an active SCSI
bus (CMOS “voltage feed-through” phenom-
ena). TolerANT technology provides signal
filtering at the inputs of SREQ/ and SACK/ to
increase immunity to signal reflections.
Differential Mode
In differential mode, the SDIR (15-0),
SDIRP0/1, IGS, TGS, RSTDIR, BSYDIR,
and SELDIR signals control the direction of
external differential-pair transceivers. The
SYM53C876 is placed in differential mode by
setting the DIF bit, bit 5 of the STEST2 reg-
ister (4Eh). Setting this bit tri-states the BSY/,
SEL/, and RST/ pads so they can be used as
pure input pins. When TolerANT active nega-
tion is enabled, the recommended resistor
value on the REQ/, ACK/, MSG/, C_D/,
I_O/, ATN/, SD15-0, and SDP1-0/signals is
1.5 K
. In addition to the standard SCSI
lines, the following signals are used during dif-
ferential operation by the SYM53C876:
See Figure 2-5 for an example differential wir-
ing diagram, in which the SYM53C876 is
connected to the TI SN75976A differential
transceiver. The recommended value of the
pull-up resistor on the REQ/, ACK/, MSG/,
C/D/, I/O/, ATN/, SD0-15/, SDP0/, and
SDP1/ lines is 680 when the Active Nega-
tion portion of Symbios Logic TolerANT
technology is not enabled. When TolerANT is
enabled, the recommended resistor value on
the REQ/, ACK/, SD7-0/, and SDP0/ signals
is 1.5 K
.
The electrical characteristics of
these pins change when TolerANT is enabled,
permitting a higher resistor value.
To interface the SYM53C876 to the
SN75976A, connect the DIR pins, as well as
IGS and TGS, of the SYM53C876 directly to
the transceiver enables (nDE/RE/). These sig-
nals control the direction of the channels on
the SN75976A.
Signal
Function
BSYDIR, SELDIR,
RSTDIR
Active high signals used to enable the differential drivers as outputs for SCSI signals BSY/,
SEL/, and RST/, respectively
SDIR(15-0),
SDIRP(0/1)
Active high signals used to control direction of the differential drivers for SCSI data and par-
ity lines, respectively
IGS
Active high signal used to control direction of the differential driver for initiator group sig-
nals ATN/ and ACK/
TGS
Active high signal used to control direction of the differential drivers for target group signals
MSG/, C/D/, I/O/, and REQ/
DIFFSENS
Input to the
SYM53C876
used to detect the presence of a single-ended device on a differ-
ential system. If a logical zero is detected on this pin, then it is assumed that a single-ended
device is on the bus and all SCSI outputs are tri-stated to avoid damage to the transceiver.