CN-0416

Circuit Note

Rev. 0 | Page 6 of 9

True Fail-Safe Receiver Inputs

In multinode systems, a bus idle condition occurs when all

nodes are in receive mode. In the bus idle condition, the

differential input voltage is 0 V. Previous generation RS-485

receivers required external fail-safe termination and biasing to

prevent the bus idle condition from producing erroneous data.

Both ADM2682E and LTC2865 tolerate bus idle conditions

without external circuitry.

The ADM2682E has true fail-safe receiver inputs with a

differential input threshold voltage to −200 mV to −30 mV.

Thus, the 0 V differential input voltage is received as a defined

logic level and prevents random erroneous data from entering

or impacting the system.

When a device requires an adjustment to the input threshold

voltage, the adjustment causes duty cycle asymmetry at the

receiver output, which worsens at low input signal levels and

slow input edge rates. The LTC2865 has a full fail-safe operation

without adjusting the input threshold voltage. Instead of

adjusting the input threshold voltage, the LTC2865 uses an

internal window comparator to determine if the input voltage

falls between the positive and negative thresholds. If this fail-

safe condition persists for more than 3 μs, the fail-safe condition

asserts and the signal is forced to a high state.

Signal and Power Isolation

The CN-0416 circuit can be configured via Switch S2 to use

either isolated communications through the ADM2682E or

nonisolated communications through the LTC2865.

When using a straight-through Ethernet cable to connect an

isolated Ethernet jack to a nonisolated Ethernet jack between

two slave nodes, connect P4 to P24 and connect P5 to P6. In

contrast, when connecting between a master and a slave node in

the same situation, connect P4 to P6 and connect P5 to P24.

When the same electrical system is used to power the supplies

of all nodes, connecting them all to the same earth ground may

reduce the noise produced by ground currents flowing between

nodes at different ground potentials. For the single earth

ground configuration, the nonisolated RS-485 communication

is most applicable.

When different nodes are powered by different electrical

systems, especially when they are situated in different buildings,

may increase the impedance of the earth ground, which in turn,

increases the likelihood of ground currents between nodes. For

potential ground current situations, isolated RS-485 communi-

cation is required. The ADM2682E has 5 kV rms signal isolation

using an Analog Devices, Inc., iCoupler® data channel. The

ADM2682E also has 5 kV rms power isolation implemented

using an Analog Devices isoPower®, integrated, isolated dc-to-

dc converter.

COMMON VARIATIONS

The RS-485 lines present in the Ethernet port are the driver and

receiver inverting and noninverting inputs, as well as supply

and ground lines. Alternative designs use a regular screw

terminal for easy wiring of standard RS-485 cables and also

include a connection for the shield.

The EVAL-CN0416-ARDZ board has a connector compatible

with the ADALM-UARTJTAG programming board to easily

interface with a PC. Furthermore, the USB-to-serial converter

can be selected to have a 3.3 V supply output, achieving current

requirements of the ADM2682E and LTC2865 that eliminate

the need for the ADP7102 low dropout regulator. The on-board

hex switch can also be removed to reduce cost and the number

of general-purpose inputs/outputs (GPIOs), using instead, a full

software implementation for node addressing.

CIRCUIT EVALUATION AND TEST

There are two classes of circuit evaluation or test setups for the

CN-0416, one for direct communications, and one for multi-

node networks. The direct communications setup is an end

node to end node, full-duplex connection. The multinode

network features master/slave communication between one

master and multiple slaves.

Both setups use the EVAL-CN0416-ARDZ circuit evaluation

board, the ADALM-UARTJTAG programming board, and the

EVAL-ADICUP3029. Serial terminal software is required to

send and display test messages. It is recommended to use a

serial terminal software, such as Tera Term, that automatically

reconnects after plugging or unplugging a device.

Equipment Needed

•

PC with a USB port and Windows 7 (32-bit) or higher

•

EVAL-CN0416-ARDZ circuit evaluation boards

•

ADALM-UARTJTAG boards

•

Straight-through RJ45 CAT5/CAT5E cables

•

USB to micro-USB cables

•

EVAL-ADICUP3029 with their corresponding power

supply adapter (loaded with the CN-0416 multinode full-

duplex example code)