LTC486

10

486fc

APPLICATIONS INFORMATION

Thecouplingcapacitorallowshighfrequencyenergytoflow

to the termination, but blocks DC and low frequencies. The

dividing line between high and low frequency depends on

the length of the cable. The coupling capacitor must pass

frequencies above the point where the line represents an

electrical one-tenth wavelength. The value of the coupling

capacitorshouldthereforebesetat16.3pFperfootofcable

length for 120Ω cables. With the coupling capacitors in

place, power is consumed only on the signal edges, not

when the driver output is idling at a 1 or 0 state. A 100nF

capacitor is adequate for lines up to 4000 feet in length.

Be aware that the power savings start to decrease once

the data rate surpasses 1/(120Ω

× C).

486 F11

140

RECEIVER

RX

5V

1.5k

RECEIVER

RX

5V

110

130

110

130

120

RECEIVER

RX

C

5V

100k

1.5k

Figure 11. Forcing “0” When All Drivers Are Off

Receiver Open-Circuit Fail-Safe

Some data encoding schemes require that the output of

the receiver maintains a known state (usually a logic 1)

when the data is finished transmitting and all drivers on

the line are forced into three-state. All LTC RS485 receivers

have a fail-safe feature which guarantees the output to be

in a logic 1 state when the receiver inputs are left floating

(open-circuit). However, when the cable is terminated with

120Ω, the differential inputs to the receiver are shorted

together, not left floating.

If the receiver output must be forced to a known state, the

circuits of Figure 11 can be used.

The termination resistors are used to generate a DC bias

which forces the receiver output to a known state, in this

case a logic 0. The first method consumes about 208mW

andthesecondabout8mW.Thelowestpowersolutionisto

use an AC termination with a pull-up resistor. Simply swap

the receiver inputs for data protocols ending in logic 1.

Fault Protection

All of LTC’s RS485 products are protected against ESD

transients up to ±2kV using the human body model

(100pF, 1.5kΩ). However, some applications need greater

protection. The best protection method is to connect a

bidirectional TransZorb from each line side pin to ground

(Figure 12).

A TransZorb is a silicon transient voltage suppressor that

has exceptional surge handling capabilities, fast response

time, and low series resistance. They are available from

General Semiconductor Industries and come in a variety

of breakdown voltages and prices. Be sure to pick a break-

down voltage higher than the common-mode voltage

required for your application (typically 12V). Also, don’t

forget to check how much the added parasitic capacitance

will load down the bus.

486 F12

120

DRIVER

Z

Y

Figure 12. ESD Protection