9

LTC1685

Output Short-Circuit Protection

The LTC1685 employs voltage sensing short-circuit pro-

tection at the output terminals of both the driver and

receiver. For a given input polarity, this circuitry deter-

mines what the correct output level should be. If the output

level is different from the expected, it shuts off the big

output devices. For example, if the driver input is >2V, it

expects the “A” output to be >3.25V and the “B” output to

be <1.75V. If the “A” output is subsequently shorted to a

devices and turns on a smaller device in its place (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 an

ideal transmission line, 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 LTC1685 will auto-

matically come out of short-circuit mode once the physical

short has been removed. With cable terminations with a

DC bias (such as Fast-20 and Fast-40 differential SCSI

terminators, see Figure 15), the LTC1685 will

not come

out of short-circuit mode automatically upon release of the

physical short. In order to resume normal operation, the

DE pin has to be pulsed low for at least 200ns.

High Speed Twisted Pair Transmission

Data rates up to 52Mbps can be transmitted over 100ft of

category 5 twisted pair. Figure 10 shows the LTC1685

receiving differential data from another LTC1685 trans-

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

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

shows a more stringent case of propagating a single 20ns

pulse over 100ft of category 5 UTP. Figure 12 shows a

4Mbps square wave over 1000ft of category 5 unshielded

twisted pair.

1685 F11

DRIVER

INPUT

RECEIVER

OUTPUT

10ns/DIV

2V/DIV

2V/DIV

Figure 11a. 100ft of Category 5 UTP: 50Mbps

1685 F11b

RECEIVER

INPUT

DRIVER

INPUT

RECEIVER

OUTPUT

20ns/DIV

2V/DIV

5V/DIV

2V/DIV

CABLE DELAY

Figure 11b. 100ft of Category 5 UTP: 20ns Pulse

Figure 10

100

Ω

A 1

4

EN

RO

1685 F10b

1/4 LTC1518

LTC1685

LTC1685

12

3

2 B

7

6

3

2

DE

DI

RO

RE

4

1

100

Ω

7

6

3

2

DE

DI

RO

RE

4

1

EN