LTC1535

13

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APPLICATIONS INFORMATION

High Voltage Considerations

The LTC1535 eliminates ground loops on data commun-

ication lines. However, such isolation can bring potentially

dangerous voltages onto the circuit board. An example

would be accidental faulting to 117V AC at some point

on the cable which is then conducted to the PC board.

Figure 12 shows how to detect and warn the user or

installer that a voltage fault condition exists on the twisted

pair or its shield. A small (3.2mm) glow lamp is connected

between GND2 (the isolated ground) and the equipment’s

safety “earth” ground. If a potential of more than 75V

AC is present on the twisted pair or shield, B1 will light,

indicating a wiring fault. Resistors R3 and R4 are used

to ballast the current in B1. Two resistors are necessary

because they can only stand off 200V each, as well as for

power dissipation. As shown, the circuit can withstand a

direct fault to a 440V 3-phase system.

Other problems introduced by floating the twisted pair

include the collection of static charge on the twisted pair,

its shield and the attached circuitry. Resistors R1 and R2

provide a path to shunt static charge safely to ground.

Again, two resisitors are necessary to withstand high

voltage faults. Electrostatic spikes, electromagnetically

induced transients and radio frequency pickup are shunted

by addition capacitor C1.

Receiver Inputs Fail-Safe

The LTC1535 features an input common mode range

covering the entire RS485 specified range of –7V to 12V.

Differential signals of greater than ± 200mV within the

specified input common mode range will be converted

to TTL compatible signals at the receiver outputs, RO

and RO2. A small amount of input hyteresis is included

to minimize the effects of noise on the line signals. If

the receiver inputs are floating or shorted, a designed-in

receiver offset guarantees a fail-safe logic high at the

receiver outputs. If a fail-safe logic low is desired, connect

as shown in Figure 19.

TWISTED-PAIR

NETWORK

2

EQUIPMENT SAFETY GROUND

EARTH GROUND

2

2

FLOATING RS485 COMMON

*

IRC WCR1206

** IRC WCR1210

*** PANASONIC ECQ-U2A103MV

B1

CN2R (JKL)

1535 F12

R1*

470k

R2*

470k

R3**

100k

R4**

100k

C1***

10nF

2

Y

A

B

Z

GND2

LTC1535

Figure 12. Detecting Fault Conditions