Function codes Modbus: exam questions with worked answers

Questions for the "Function codes" topic

1. Q01

   Which Modbus function code is used to READ the Holding Registers (16-bit read/write registers) of a slave?

   • A.0x01 (Read Coils)
   • B.0x02 (Read Discrete Inputs)
   • C.0x03 (Read Holding Registers)
   • D.0x04 (Read Input Registers)

   4. Function codes· Lesen Coils/Register· Medium

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   Correct answer

   C — 0x03 (Read Holding Registers)

   Learning tip

   0x03 (Read Holding Registers) targets the 16-bit read/write register space and is by far the most used code for reading analogue values such as temperatures, pressures, counters and setpoints. 0x01 is wrong because Read Coils returns 1-bit read/write outputs, not 16-bit registers. 0x02 is wrong because Read Discrete Inputs returns 1-bit read-only digital inputs. 0x04 is wrong because Read Input Registers targets a separate 16-bit READ-ONLY register table, not the Holding Registers. The full picture includes 0x05/0x06 for single writes and 0x0F/0x10 for multiple writes.

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2. Q02

   Function 0x06 "Write Single Register" writes a single 16-bit Holding Register on a Modbus slave, with both the address and the value encoded on 16 bits each in the request.

   TrueFalse

   4. Function codes· Write Single Register (0x06)· Easy

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   Correct answer

   True

   Learning tip

   The 0x06 request format is: function code on 1 byte (0x06), register address on 2 bytes (0-65535), and register value on 2 bytes (0-65535) -- 5 bytes of PDU plus the transport headers. The dual function code is 0x10 "Write Multiple Registers", which writes several consecutive registers in a single frame, up to a limit of 123 registers per request.

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