9

LTC1686/LTC1687

(the converse applies for the “B” output). The outputs then

appear as

±10mAcurrentsources.Notethatundernormal

operation, the output drivers can sink/source >50mA. A

time-out period of about 50ns is used in order to maintain

normal high frequency operation, even under heavy ca-

pacitive loads.

If the cable is shorted at a large distance from the device

outputs, it is possible for the short to go unnoticed at the

driver outputs due to parasitic cable resistance. Addition-

ally, when the cable is shorted, it no longer appears as a

simple transmission line impedance, and the parasitic L’s

and C’s might give rise to ringing and even oscillation. All

these conditions disappear once the device comes out of

short-circuit mode.

For cables with the typical RS485 termination (no DC bias

on the cable, such as Figure 10), the LTC1686/LTC1687

will automatically come out of short-circuit mode once the

physical short has been removed.

Cable Termination

The recommended cable termination for the LTC1686/

LTC1687 is a single resistor across the two wires at each

end of the twisted-pair line (see Figure 10). The LTC1687

can also be used with cable terminations with a DC bias

(such as Fast-20 and Fast-40 differential SCSI termina-

tors). When using a biased termination with the LTC1687,

however, the DE pin must be held low for at least 200ns

after the part has been powered up. This ensures proper

start-up into the DC load of the biased termination. Fur-

thermore, when the LTC1687 output is shorted, the DE pin

should be pulsed low for at least 200ns after the short has

been removed. Since the LTC1686 driver is always

enabled, the LTC1686 should only be used with single

resistor termination, as shown in Figure 10.

HIGH SPEED TWISTED-PAIR TRANSMISSION

Data rates up to 52Mbps can be transmitted over 100 feet

of category 5 twisted pair. Figure 10 shows the LTC1687

receiving differential data from another LTC1687 trans-

ceiver. Figure 11a shows a 26MHz (52Mbps) square wave

propagated over 100 feet of category 5 UTP. Figure 11b

shows a more stringent case of propagating a 20ns pulse

over 100 feet of category 5 UTP. Figure 12 shows a 2MHz

(4Mbps) square wave propagated over 1000 feet of

category 5 unshielded twisted pair. Note that the LTC1686/

LTC1687 can still perform reliably at this distance and

speed. Very inexpensive unshielded telephone grade

twisted pair is shown in Figure 13. Despite the noticeable

loss at the receiver input, the LTC1686/LTC1687 can still

transfer at 30Mbps over 100 feet of telephone grade UTP.

Note that under all these conditions, the LTC1686/LTC1687

can pass through a single data pulse equal to the inverse

of the data rate (e.g., 20ns for 50Mbps data rate).

TRANSMISSION OVER LONG DISTANCES

1Mbps Over 4000 Feet Category 5 UTP

The LTC1685/LTC1686/LTC1687 family of high speed

transceivers is capable of 1Mbps transmission over 4000

feet of category 5 UTP. High quality cable provides lower

Figure 10

LTC1686/87 • F10

100

Ω

100

Ω

100

Ω

100

Ω

LTC1687

R

D

R

D

5

2

11

12

10

9

4

DE

3

RE

DE

RE

CATEGORY 5 UTP

RECEIVER

LTC1687

DRIVER

RECEIVER

DRIVER