
2-20
SYM53C876/876E Data Manual
Functional Description
SCSI Functional Description
Terminator Networks
The terminator networks provide the biasing
needed to pull signals to an inactive voltage level,
and to match the impedance seen at the end of the
cable with the characteristic impedance of the
cable. Install terminators at the extreme ends of
the SCSI chain, and only at the ends; no system
should ever have more or less than two termina-
tors installed and active. SCSI host adapters
should provide a means of accommodating termi-
nators. The terminators should be socketed, so
that if not needed they may be removed, or there
should be a means of disabling them with soft-
ware.
Single-ended cables can use a 220
pull-up to
the terminator power supply (Term-Power) line
and a 330
pull-down to Ground. Because of the
high-performance nature of the SYM53C876,
Regulated (or Active) termination is recom-
mended. Figure 2-6 shows a Unitrode active
terminator. For additional information, refer to
the SCSI-2 Specification. TolerANT active nega-
tion is compatible with either termination net-
work.
Note: If the SYM53C876 is used in a design
with only an 8-bit SCSI bus, all 16 data
lines still must be terminated or pulled
high.
Note: Active termination is required for Wide
Ultra SCSI synchronous transfers.
Synchronous Operation
The SYM53C876 can transfer synchronous SCSI
data in both initiator and target modes. The
SXFER register controls both the synchronous
offset and the transfer period. It may be loaded by
the CPU before SCRIPTS execution begins, from
within SCRIPTS via a Table Indirect I/O instruc-
tion, or with a Read-Modify-Write instruction.
Key
C1
C2
C3
J1
10
μ
F SMT
0.1
μ
F SMT
2.2
μ
F SMT
68-pin, high density “P” connector
Figure 2-6: Regulated Termination
UC5601QP
C1
C2
TERML1
TERML2
TERML3
TERML4
TERML5
TERML6
TERML7
TERML8
TERML9
TERML10
TERML11
TERML12
TERML13
TERML14
TERML15
TERML16
TERML17
TERML18
20
21
22
23
24
25
26
27
28
3
4
5
6
7
8
9
10
11
SD0 (J1.40)
SD1 (J1.41)
SD2 (J1.42)
SD3 (J1.43)
SD4 (J1.44)
SD5 (J1.45)
SD6 (J1.46)
SD7 (J1.47)
SDP0 (J1.48)
ATN (J1.55)
BSY (J1.57)
ACK (J1.58)
RST (J1.59)
MSG (J1.60)
SEL (J1.61)
C/D (J1.62)
REQ (J1.63)
I/O (J1.64)
REG_OUT
DISCONNECT
2.85V
2
19
TERM1
TERM2
TERM3
TERM4
TERM5
TERM6
TERM7
TERM8
TERM9
10
9
8
7
3
2
1
16
15
SD15 (J1.38)
SD14 (J1.37)
SD13 (J1.36)
SD12 (J1.35)
SD11 (J1.68)
SD10 (J1.67)
SD9 (J1.66)
SD8 (J1.65)
SDP1 (J1.39)
UC5603DP
6
DISCONNECT
14
REG_OUT
C3