MODBUS Protocol Questions and Answers
- What is the most common physical layer used for Modbus communication?
a) RS-232
b) RS-485
c) Ethernet
d) USB
Answer: b) RS-485
2. Which function code is used to read the contents of a holding register in Modbus?
a) 01
b) 02
c) 03
d) 04
Answer: d) 04
3. What is the maximum number of devices that can be connected to a single Modbus network?
a) 32
b) 64
c) 128
d) 256
Answer: d) 256
4. What is the default baud rate for Modbus RTU?
a) 9600
b) 19200
c) 38400
d) 57600
Answer: b) 19200
5. What is the maximum data payload size for a Modbus TCP packet?
a) 260 bytes
b) 512 bytes
c) 1024 bytes
d) 2048 bytes
Answer: a) 260 bytes
6. What is the Modbus function code for writing a single coil?
a) 05
b) 06
c) 15
d) 16
Answer: b) 06
7. What is the Modbus function code for reading input registers?
a) 04
b) 05
c) 06
d) 03
Answer: d) 03
8. How many data bytes can be written to a single holding register using Modbus function code 16?
a) 1
b) 2
c) 4
d) 8
Answer: b) 2
9. Is Modbus protocol a master-slave or peer-to-peer protocol?
a) Master-slave
b) Peer-to-peer
Answer: a) Master-slave
10. Which Modbus protocol variant uses ASCII character encoding?
a) Modbus RTU
b) Modbus ASCII
Answer: b) Modbus ASCII
11. Which Modbus function code is used to read the status of discrete inputs?
a) 01
b) 02
c) 03
d) 04
Answer: c) 03
12. What is the maximum number of data bytes that can be read in a single Modbus function code 03 request?
a) 1
b) 2
c) 125
d) 256
Answer: c) 125
13. How is the Modbus slave address specified in a Modbus RTU packet?
a) As a 2-byte value
b) As a single byte
c) As a 4-byte value
d) As a 6-byte value
Answer: b) As a single byte
14 . What is the default value of the transaction identifier in a Modbus TCP packet?
a) 0
b) 1
c) 2
d) 3
Answer: b) 1
15. What is the Modbus function code for writing multiple holding registers?
a) 16
b) 15
c) 14
d) 13
Answer: b) 15
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