SCADA / DCS Engineer Interview question : Troubleshooting of Modbus Communication


#1

How to troubleshoot Modbus communication problem between DCS / SCADA and third party devices.

We may see two types of major error while dealing with modbus communication.

  1. No response error
  2. CRC error

The causes of No response error are

  1. Modbus parameters of Master and Slave may be different
    Modbus parameters are Baud rate,Frame Format (ASCII / RTU) ,Start Bit , Stop bit ,Parity
    2.Incorrect address of slave
    3.Check the physical connection (Wiring and power )
    4.Check the reverse polarity of RS485 line
    5.Ensure slave device is enabled for Modbus communication

CRC error

1.Ensure baud rate and character format are correct
2.Check physical connection (Wiring) .Noise may be the culprit
3.Check reversed polarity on RS485 lines


#2

Types of Register used in Modbus

Holding Register - An area of 16 bit words . Starts from 40000
Input Registers - 16 bit words .Analog inputs .Starts from 30000
Inputs - Binary Input .Starts from 10000
Coil -Binary output .Starts from 20000


#3

Hello Denizen,

I have 2 questions need support:

  1. Can we transfer/receive Real data (32 bit) thru Modbus protocol?
  2. Some SCADA/Metering Supervisor Computer transeive Real Data by separating it to 2 parts (High Byte Int (16bit) & Low byte Int (16bit)). How to know that system follow Little Endian or Big Endian?

Thanks,
Vu,


#4

Yes, We can trasfer 32 bit data through modbus protocol.Some devices using special adresses from 70000 to read the 32 bit directly.
If you are using 16 bit registers then you need to write program for merging two 16 bit registers.


#5

Sufficient answer and I want to know about VSAT


#6

I want to know about VSAT and modbus protocol