The recommended cable termination for use with the
LTC1688/LTC1689 is a single resistor across the two ends
of a transmission cable (see Figure 8). When PC traces are
used as the transmission line, its characteristic imped-
ance should be chosen close to 100Ω in order to better
match the specified timing characteristics of the LTC1688/
LTC1689. Category 5 unshielded twisted pair can be used
over short distances at the maximum data rates (100Mbps).
For point-to-point configurations (see Figure 9), a single
resistor across the cable at the receiver end is sufficient.
A single resistor termination lowers power consumption
and increases the differential output signal. See Enable
Pins section for cable terminations with a DC bias.
For cable terminations with a DC bias (such as High
Voltage Differential SCSI, see Figure 10), the driver out-
puts must be disabled for at least 200ns after power-up.
This ensures that the driver outputs do not disturb the
cable upon power-up. It also ensures the correct output
start-up conditions. When there is an output short fault
condition and the cable has a DC biased termination, such
as Figure 10, the driver outputs must be disabled for at
least 200ns after the short has been removed. Recall that
for transmission lines that have the recommended RS485
single resistor termination (Figures 8 and 9), the LTC1688/
LTC1689 will come out of a short-circuit fault condition
automatically without having to disable the outputs.
Figure 10. DC-Biased Termination
(Recommended for SCSI Applications Only)
Figure 8. Multipoint Transmission Figure 9. Point-to-Point Transmission