Circuit Note

CN-0416

Rev. 0 | Page 3 of 9

Table 1. RS-485 Bus Standard Specifications

Specification

Standard

ADM2682E

LTC2865

Data Rate

10 Mbps

16 Mbps

20 Mbps

Driver Output Applied Voltage Range

−7 V to +12 V

−9 V to +14 V, isolated

−60 V to +60 V

Receiver Input Voltage Range

−7 V to 12 V

−9 V to +14 V, isolated

−60 V to +60 V

Receiver Input Sensitivity

± 200 mV

−200 to −30 mV

±200 mV

Cable Length

Up to 1200 m

Up to 1200 m

Up to 1200 m

Receiver Input Resistance

≥12 kΩ

96 kΩ

112 kΩ

Number of Drivers and Receivers

32 drivers

256 drivers and receivers

256 drivers and receivers

32 receivers

The inclusion of the isolated and nonisolated transceivers on

EVAL-CN0416-ARDZ aids in determining the actual

requirements and implementation of the end application.

The circuit has four on-board Ethernet jacks, one pair for

isolated and one pair for nonisolated communications. Whereas

Ethernet cable is not intended for RS-485 applications, and the

impedance is 100 Ω vs 120 Ω for RS-485, the ubiquity of

common CAT5 and CAT6 cables provides a convenient way to

build prototype systems during development. Also, each pair of

Ethernet ports have crossover pin configurations, allowing the

use of straight-through Ethernet cables for multinode systems.

Connection points to the RS-485 signals are provided,

facilitating the use of other connectors, or direct wiring to

theRS-485 cable of the application.

RS-485 Bus Node Capacity

Both the ADM2682E and LTC2865 have 1/8 unit loading of

96 kΩ minimum, allowing up to 256 nodes in a single bus

system. (The RS-485 standard specifies a receiver input

impedance of 12 kΩ and a maximum of 32 nodes.)

Termination

A common guideline for termination is to terminate lines

transmitting signals with rise/fall times less than four times

the propagation delay of the cable. A standard CAT5 cable has a

propagation delay of 4.8 ns/m to 5.3 ns/m. The ADM2682E and

LTC2865 have maximum rise/fall times of 15 ns. Thus, for lines

greater than 0.78 m in length, it is recommended to use termi-

nation. For best practice, properly terminate all RS-485 systems.

Figure 3. Termination Selection Switches

The circuit shown in Figure 3 can configure the RS-485 line

termination to either a differential resistance or left open via S7

for isolated communications and via S6 for nonisolated

communications. The full-duplex RS-485 standard requires a

termination at the master node and the farthest slave node.

For a half-duplex connection, terminate both ends of the

transmission cable. The value of the termination resistance

should be equal to the characteristic impedance of the cable.

For CAT5 cables, this is 100 Ω. By default, the circuit uses a

100 Ω termination resistance but can be configured through

solder links to use the standard RS-485 termination resistance,

which is 120 Ω. Table 2 shows the advantages and disadvantages

of open and terminated RS-485 communications.

Table 2. Advantages and Disadvantages of Termination

Termination

Advantages

Disadvantages

Open or No

Termination

Simple, low power

For slow and short

communications

Parallel

Resistance

Supports long

distance and fast

communications

High power

Half Duplex Operation

Figure 4. Half- or Full-Duplex Selection Switches

The circuit shown in Figure 4 can be set to operate in half-

duplex mode for both isolated and nonisolated communications

via Switch S5 and Switch S4, respectively. The driver

noninverting output is shorted with the noninverting input of

S6 OR S7

DPDT

SWITCH

R9

100Ω

R8

120Ω

Z

R11

100Ω

R10

120Ω

B

Y

A

YOUT

AOUT

ZOUT

BOUT

YOUT

YOUT

S4 OR S5

4PDT

SWITCH

A

B

Y

Z