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Communication

Grounding and Shielding

In most cases, I/O modules will be installed in an enclosure along with the other devices, which generate electromagnetic radiation. Relays, contactors, transformers, motor invertors, etc., are examples of such devices. Radiation can induce electrical noise into both power and signal lines, as well as direct radiation into the module. Whether or not the SfAR modules are immune to such effects, the interferences must be suppressed at their source if possible to ensure the proper functioning of the entire system. Appropriate grounding, shielding and other protective steps should be taken at the installation stage to prevent these effects. It is recommended to at least follow the rules below:

  • line power cables must be routed with spatial separation from signal and data transmission cables;

  • analog and digital signal cables should also be separated;

  • it is recommended to use shielded cables for analog signals, cable shields should not be interrupted by intermediate terminals;

  • the shielding should be earthed directly after the cable enters the cabinet.

It is recommended to install interference suppressors when switching inductive loads (e.g., coils of contactors, relays, solenoid valves). RC snubbers or varistors are suitable for AC voltage and freewheeling diodes for DC voltage loads. The suppressing elements must be connected as close to the coil as possible.

Network Termination

Transmission line effects often present problems for data communication networks. These problems include reflections and signal attenuation. To eliminate the presence of reflections of signal from the end of the cable, the cable must be terminated at both ends with a resistor across the line adequate to its characteristic impedance. Both ends must be terminated since the propagation is bidirectional. In case of an RS485 twisted pair cable, this termination is typically 120 Ω.

Types of Modbus Functions

There are 4 types of Modbus functions supported by the SfAR modules.

Type

Beginning Address

Variable

Access

Modbus Command

1

00001

Digital Outputs

Bit
Read/write

1, 5, 15

2

10001

Digital Inputs

Bit
Read

2

3

30001

Input Registers

Registered
Read

3

4

40001

Output Registers

Registered
Read/write

4, 6, 16

Types of Modbus functions supported by the module

Communication Settings

The data stored in the module's memory is given in the 16-bit registers. The access to registers is via Modbus RTU or Modbus ASCII.

Default Settings

Parameter Name

Value

Address

1

Baud rate

19200

Parity

No

Data bits

8

Stop bits

1

Reply delay [ms]

0

Modbus type

RTU

Default settings

Configuration Registers

Modbus Address

Decimal Address

Hex Address

Name

Values

40003

2

0x02

Baud Rate

0 – 2400
1 – 4800
2 – 9600
3 – 19200
4 – 38400
5 – 57600
6 – 115200
other – value * 10

40005

4

0x04

Parity

0 – none
1 – odd
2 – even
3 – always 0
4 – always 1

40004

3

0x03

Stop Bits

1 – one stop bit
2 – two stop bits

40004

3

0x03

Data Bits

7 – 7 data bits
8 – 8 data bits

40006

5

0x05

Response Delay

Time in ms

40007

6

0x06

Modbus Mode

0 – RTU
1 – ASCII

Configuration registers

Watchdog

This 16-bits register specifies the time in milliseconds to watchdog reset. If the module does not receive any valid message within that time, all digital and analog outputs will be set to the default state.

This feature is useful if there is an interruption in data transmission and for security reasons. Output states must be set to the appropriate state in order to reassure the safety of persons or property.

The default value is 0 milliseconds, which means the watchdog function is disabled.


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