8

LTC1688/LTC1689

Cable Termination

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 100

Ω 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.

Enable Pins

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.

1688/89 F10

1/4 LTC1518

150

Ω

330

Ω

DE

DI

330

Ω

1/4 LTC1688

TERM POWER

150

Ω

330

Ω

330

Ω

TERM POWER

1/4 LTC1518

Figure 10. DC-Biased Termination

(Recommended for SCSI Applications Only)

Figure 8. Multipoint Transmission

Figure 9. Point-to-Point Transmission

1688/89 F08

1/4 LTC1518

1/4 LTC1519

100

Ω

100

Ω

1/4 LTC1688

1688/89 F09

1/4 LTC1518

100

Ω

1/4 LTC1689