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ADM2461E/ADM2463E

Data Sheet

Rev. 0 | Page 18 of 21

DRIVER AND RECEIVER CABLE INVERSION

The ADM2461E/ADM2463E feature cable inversion

functionality to correct for errors during installation. This

adjustment can be implemented in the software on the controller

driving the RS-485 transceiver to avoid installation costs to fix

wiring errors. The ADM2463E full duplex transceiver features a

receiver cable invert pin, INVR, that can be used to correct

receiver functionality in cases where connections to the A and

B pins are made in reverse. The ADM2461E half-duplex

transceiver features a single cable invert logic input pin, INV, that

inverts the driver and receiver to correct for reversed connections

to the A and B pins. When the receiver is inverted, the device

maintains a Logic 1 receiver output with a 30 mV noise margin

when inputs are shorted together or open-circuit. Figure 42

shows the receiver output in inverted and noninverted cases.

+30mV

PHASE INVERTED RS-485

INVR = H

NONINVERTED

RS-485

INVR = L

+200mV

–30mV

–200mV

A – B

FAIL-SAFE

Figure 42. Noninverted RS-485 and Phase Inverted RS-485 Comparison

HOT SWAP INPUTS

When a circuit board is inserted into a powered (or hot)

backplane, parasitic coupling from supply and ground rails to

digital inputs can occur. The ADM2461E/ADM2463E contain

circuitry to ensure that the RS-485 driver outputs remain in a

high impedance state during power-up, and then default to the

correct states. For example, when VDD1 and VDD2 power up at the

same time and the RE pin is pulled low with the DE and TxD pins

pulled high, the A and B outputs remain in high impedance until

the outputs settle at an expected default high for the A pin and

expected default low for the B pin.

192 TRANSCEIVERS ON THE BUS

The standard RS-485 receiver input impedance is 12 kΩ (1 unit

load) and the standard driver can drive up to 32 unit loads.

The ADM2461E/ADM2463E transceivers have a 1/6 unit

load receiver input impedance (equivalent to 72 kΩ) that

allows up to 192 transceivers to be connected in parallel on one

communication line. Any combination of these devices and

other RS-485 transceivers with a total of 32 unit loads or fewer can

be connected to the line.

DRIVER OUTPUT PROTECTION

The ADM2461E/ADM2463E have two methods to prevent

excessive output current and power dissipation caused either by

faults or by bus contention. Current-limit protection on the output

stage provides immediate protection against short circuits over the

entire common-mode voltage range. In addition, a thermal

shutdown circuit forces the driver outputs into a high impedance

state if the die temperature rises excessively. This circuitry disables

the driver outputs when a die temperature of 150°C is reached.

As the device cools, the drivers are re-enabled at a temperature

of 140°C.

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