Vcc

GND

B

A

D

DE

R

RE

a) Independent driver and

receiver enable signals

b) Combined enable signals for

use as directional control pin

c) Receiver always on

R

D

Vcc

GND

B

A

D

DE

R

RE

R

D

Vcc

GND

B

A

D

DE

R

RE

R

D

SN888C

www.ti.com

SLLSEI4 – SEPTEMBER 2013

APPLICATION INFORMATION

Device Configuration

The SN888C device is a half-duplex RS-485 transceiver operating from a single 5-V ±10% supply. The driver

and receiver enable pins that allow for the configuration of different operating modes.

Figure 13. Transceiver Configurations

Using independent enable lines provides the most flexible control as the lines allow for the driver and the

receiver to be turned on and off individually. While this configuration requires two control lines, it allows for

selective listening to the bus traffic, whether the driver is transmitting data or not. Only this configuration allows

the SN888C device to enter low-power standby mode because it allows both the driver and receiver to be

disabled simultaneously.

Combining the enable signals simplifies the interface to the controller by forming a single direction-control signal.

Thus, when the direction-control line is high, the transceiver is configured as a driver, while when low, the device

operates as a receiver.

Tying the receiver enable to ground and controlling only the driver-enable input also uses only one control line. In

this configuration, a node not only receives the data on the bus sent by other nodes, but also receives the data

sent on the bus, enabling the node to verify the correct data has been transmitted.

Bus Design

An RS-485 bus consists of multiple transceivers connected in parallel to a bus cable. To eliminate line

reflections, each cable end is terminated with a termination resistor, RT, whose value matches the characteristic

impedance, Z0, of the cable. This method, known as parallel termination, allows for relatively high data rates over

long cable length.

Common cables used are unshielded twisted pair (UTP), such as low-cost CAT-5 cable with Z0 = 100

Ω, and

RS-485 cable with Z0 = 120

Ω. Typical cable sizes are AWG 22 and AWG 24.

The maximum bus length is typically given as 4000 ft or 1200 m, and represents the length of an AWG 24 cable

whose cable resistance approaches the value of the termination resistance, thus reducing the bus signal by half

or 6 dB. Actual maximum usable cable length depends on the signaling rate, cable characteristics, and

environmental conditions.

Table 1. VID With a Failsafe Network and Bus Termination

VCC

RL Differential

RFS Pull-Up

RFS Pull-Down

VID

Termination

560

Ω

560

Ω

230 mV

1 K

Ω

1 K

Ω

131 mV

5 V

54

Ω

4.7 K

Ω

4.7 K

Ω

29 mV

10 K

Ω

10 K

Ω

13 mV

An external failsafe-resistor network must be used to ensure failsafe operation during an idle bus state. When the

bus is not actively driven, the differential receiver inputs could float allowing the receiver output to assume a

random output. A proper failsafe network forces the receiver inputs to exceed the VIT threshold, thus forcing the

SN888C receiver output into the failsafe (high) state. Table 1 shows the differential input voltage (VID) for various

failsafe networks with a 54-

Ω differential bus termination.

Copyright © 2013, Texas Instruments Incorporated

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