AN2354

Connecting ASC to an RS485 network

13/21

3

Connecting ASC to an RS485 network

3.1

Introduction

An RS232 interface connects one DTE (data terminal equipment) to one DCE (data

communication equipment) at a maximum speed of 20 Kbit/s with a maximum cable length

of 30 meters. This was sufficient early on when almost all computer equipment was

connected by modems but, soon after, interfaces capable of one or more of the following

were in demand:

●

Connecting DTE's directly without the need for modems

●

Connecting several DTE's in a network structure

●

Communicating over longer distances

●

Communicating at faster speeds

RS485 is the most versatile communication standard in the standard series defined by the

EIA, as it performs well on all four points. Thus RS485 is currently a widely-used

communication interface in data acquisition and control applications where multiple nodes

communicate with each other.

3.2

RS485 versus RS232

One of the main problems with RS232 is the lack of immunity against noise on the signal

lines. The transmitter and receiver compare the voltages of the data and handshake lines

with one common zero line. Shifts in the ground level can have disastrous effects. Therefore,

the trigger level of the RS232 interface is set relatively high at ±3V (

Table 3). Noise is easily

picked up and limits both the maximum distance and communication speed. On the

contrary, a common zero as a signal reference does not exist with RS485. A difference of

several volts in the ground level of the RS485 transmitter and receiver does not cause any

problems. The RS485 signals are floating and each signal is transmitted over a B line and

an A line. The RS485 receiver compares the voltage difference between both lines,

instead of the absolute voltage level on a signal line. This works well and prevents the

existence of ground loops, a common source of communication problems. The best results

are achieved if the B and A lines are twisted.

When the cable is twisted, we see that in some parts of the signal lines the direction of the

noise current is the opposite from the current in other parts of the cable. Consequently, the

resulting noise current is many factors lower than with an ordinary straight cable. Shielding

tries to isolate the signal lines from hostile magnetic fields. Twisted pairs in RS485

communication, however, add immunity, which is a much better way to fight noise.

Differential signals and twisting allow RS485 to communicate over much longer distances

than achievable with RS232. With RS485, communication distances of 1200 meters are

possible.

Differential signal lines also allow higher bit rates than possible with non-differential

connections. Therefore, RS485 can overcome the practical communication speed limit of

RS232. Currently, RS485 drivers are produced that can attain a bit rate of 35 Mbit/s.