Communication

Grounding and Shielding

In most cases, I/O modules will be installed in an enclosure along with other devices, which generate electromagnetic radiation. Relays, contactors, transformers, motor controllers, 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
40002	1	0x01	Address	From 0 to 255
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