SfAR-1M-4DI User Manual
www.ismacontrolli.com
DMP269en | 3rd Issue rev. 1 | 05/2022
page 7 of 16
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4 Communication
4.1 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.
4.2 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.
4.3 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
