Seagate Cheetah 15K.5 SAS ST373455SS - Manual (Page 74)

68 Cheetah 15K.5 SAS Product Manual, Rev. E
9.5.2.4 Electrical TxRx connections
TxRx connections may be divided into TxRx connection segments. In a single TxRx connection individual
TxRx connection segments may be formed from differing media and materials, including traces on printed wir-
ing boards and optical fibers. This subclause applies only to TxRx connection segments that are formed from
electrically conductive media.
Each electrical TxRx connection segment shall comply with the impedance requirements of table 20 for the
media from which they are formed. An equalizer network, if present, shall be part of the TxRx connection.
TxRx connections that are composed entirely of electrically conducting media shall be applied only to homoge-
nous ground applications (e.g., between devices within an enclosure or rack, or between enclosures intercon-
nected by a common ground return or ground plane).
9.5.2.4.1 Transmitter characteristics
The drive are D.C. coupled.
Common mode impedance
b,e
ohm 20 min/40 max 20 min/40 max
Transmitter source termination
Differential impedance
b
ohm 60 min/115 max 60 min/115 max
Differential impedance imbalance
b,g
ohm 5 5
Common mode impedance
b
ohm 15 min/40 max 15 min/40 max
a All times indicated for time domain reflectometer measurements are recorded times. Recorded times are twice the
transit time of the time domain reflectometer signal.
b All measurements are made through mated connector pairs.
c The media impedance measurement identifies the impedance mismatches present in the media when terminated in
its characteristic impedance. This measurement excludes mated connectors at both ends of the media, when
present, but includes any intermediate connectors or splices. The mated connectors measurement applies only to the
mated connector pair at each end, as applicable.
d Where the media has an electrical length of > 4 ns the procedure detailed in SFF-8410, or an equivalent procedure,
shall be used to determine the impedance.
e The receiver termination impedance specification applies to all receivers in a TxRx connection and covers all time
points between the connector nearest the receiver, the receiver, and the transmission line terminator. This
measurement shall be made from that connector.
f At the time point corresponding to the connection of the receiver to the transmission line the input capacitance of the
receiver and its connection to the transmission line may cause the measured impedance to fall below the minimum
impedances specified in this table. The area of the impedance dip (amplitude as
ρ , the reflection coefficient, and
duration in time) caused by this capacitance is the receiver termination time constant. The receiver time constant
shall not be greater than the values shown in this table. An approximate value for the receiver termination time
constant is given by the product of the amplitude of the dip (as
ρ ) and its width (in ps) measured at the half amplitude
point. The amplitude is defined as being the difference in the reflection coefficient between the reflection coefficient at
the nominal impedance and the reflection coefficient at the minimum impedance point. The value of the receiver
excess input capacitance is given by the following equation:
where (R0 || RR) is the parallel combination of the transmission line characteristic impedance and
termination resistance at the receiver.
g The difference in measured impedance to ground on the plus and minus terminals on the interconnect, transmitter or
receiver, with a differential test signal applied to those terminals.
Table 20: Impedance requirements (Sheet 2 of 2)
Requirement Units 1.5 Gbps 3.0 Gbps
C
receiver termination time constant
R 0 RR | ( )
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