Serial Communications

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Introduction

Many PLC devices support Asynchronous Serial Communications, where data is sent 1 bit at a time.

The structure of data sent serially is:

Start Bit5 - 8 Data bitsOptional Parity bit1 - 2 Stop Bits

Within this structure there are a number parameters:

To successfully connect two devices using serial communications, both devices must use the same set of parameters.

Baud Rate

The Baud Rate is the speed at which data bits are sent. Common values for the Baud Rate used in PLC Devices include: 1200, 9600, 19200, 38400.

Data Bits

The Data Bits is the number of bits of data in each frame. Common values used by PLC Devices include 7 or 8 data bits. All binary protocols use 8 data bits. ASCII data can be sent using 7 or 8 bits, for example Modbus ASCII.

Parity

The Parity bit can provide a simple form of error detection:

ParityDescription
NoneNo parity bit is added to the data.
EvenThe parity bit is set to space (0) if the total number of data bits in the mark (1) state is even.
OddThe parity bit is set to space (0) if the total number of data bits in the mark (1) state is odd.
MarkThe parity bit is set to mark (1).
SpaceThe parity bit is set to space (0).

Note: Only Even and Odd parity provide a level of error detection.

When a data byte includes Even or Odd parity, the receiver can detect single bit errors. The receiver calculates the expected parity bit for the received data. If the received parity bit does not match the expected parity bit, one or more bits have been corrupted. If a bit error is detected, the receiver can ignore the corrupted data byte.

Stop Bits

The Stop Bits is the number of bits used to mark the end of a frame. Common values used by PLC Devices are 1 or 2 stop bits.

Electrical Signals

Individual bits are represented by electrical signals. There are a number of standards which define the electrical signals that represent a mark (1) bit or a space (0) bit:

StandardModeMarkSpace
RS-232 Single Ended -3v to -15v +3v to +15v
RS-422 Differential 0v to -6v 0v to +6v
RS-485 Differential -0.2v to -6v +0.2v to +6v

Single Ended mode uses a single wire where the voltage is measured with respect to ground. Differential mode uses a pair of wires where the voltage is the difference between the two wires. Single Ended mode can be used over short runs of up to 15 metres. Differential mode can work up to 1500 metres.

RS-422/RS-485 Convert Guides

Very few computers have a built-in serial RS-422/RS-485 port. Common ways of providing an RS-422/RS-482 serial ports include:

For information on how to use RS-422/RS-485 adapters see the following guides:

Further Information

Serial Port Mode String

To learn about configuring serial port mode strings in Fernhill SCADA.

Serial Port Mode Editor

To learn about the editor that can edit serial port mode strings.