CN-0416

Circuit Note

Rev. 0 | Page 2 of 9

Figure 1. Isolated and Nonisolated RS-485 Transceiver Simplified Schematic

CIRCUIT DESCRIPTION

RS-485 Bus Standard

The RS-485 bus standard is one of the most commonly used

physical layers in industrial applications needing local networks

capable of multidrop communication links and long haul data

transfer over distances up to 1200 m. A data rate of 10 Mpbs is

achievable for cable lengths up to 12 m, dropping to 100 kbps at

1200 m, as shown in Figure 2.

Figure 2. Cable Length vs. Data Rate

Table 1 shows the some of the specifications of the RS-485 bus

standard, as well as the improved specifications that the

ADM2682E and LTC2865 provide.

For the full details of each transceiver device, refer to their data

sheets. The extended specifications of the ADM2682E and

LTC2865 improve the robustness of systems that are nominally

compliant but may occasionally experience events that break

the standard. For example, two equipment cabinets in a factory

may both be tied to earth ground, with a few volts ac between

grounds. However, an ESD strike to one cabinet may

momentarily drive its common-mode voltage to 20 V. An

LTC2865 rides through such an event, while a transceiver that

just meets the standard may produce corrupt data or fail

entirely.

The ADM2682E greatly extends common-mode tolerance,

allowing for 25 kV/μs transient immunity, with up to 5000 V

potential difference between devices (1 minute, per UL1577).

Refer to the ADM2682E data sheet for details on safety and

regulatory approvals.

3V3D

R2

10kΩ

VCC

TxD

SWITCH POSITIONS

1: FULL DUPLEX

2: HALF DUPLEX

S4

4PDT SWITCH

S7

DPDPT

SWITCH

S6

DPDPT

SWITCH

U1

ADM2682E

U3

LTC2865

RxD

DE

RE

GND1

DE

RE

DI

DO

SLO

VISOOUT

VISOIN

B

R12

120Ω

R14

120Ω

R13

100Ω

R15

100Ω

Z

Y

A

GND2

VCC

VL

GND

A

B

Y

Z

GND

GND

GND_ISO

GND_ISO

P5

R145

PORT

P4

R145

PORT

P6

R145

PORT

P24

R145

PORT

GND

8

7

6

5

4

3

2

1

8

7

6

5

4

3

2

1

SWITCH POSITIONS

1: FULL DUPLEX

2: HALF DUPLEX

S5

4PDT SWITCH

SWITCH POSITIONS

1: ISOLATED HALF DUPLEX

2: ISOLATED FULL DUPLEX

3: NON-ISOLATED FULL DUPLEX

4: NON-ISOLATED HALF DUPLEX

S2

4P4T SWITCH

B

R8

120Ω

R11

120Ω

R9

100Ω

R10

100Ω

Z

Y

A

A

B

Y

Z

8

7

6

5

4

3

2

1

8

7

6

5

4

3

2

1

VISO

Tx

IOREF

Rx

5V_ARD

V_ISO

IOREF

Tx

Rx

3V3D

10k

1k

100

10

10k

50k

100k

500k

DATA RATE (bps)

1M

5M

10M

RS485

SPECIFIED LIMITS