Modbus Protocol –Technical description
Modbus Protocol
Technical description
Version 1.0
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Modbus Protocol –Technical description
1. Index
1. Index ............................................................................................................................................................................................... 2
2. Preface ............................................................................................................................................................................................ 3
2.1. Overview .................................................................................................................................................................................... 3
2.2. Default settings .......................................................................................................................................................................... 3
3. Modbus commands ......................................................................................................................................................................... 4
3.1. Read holding registers (function code 03) .................................................................................................................................. 4
3.1.1. Frame layout .................................................................................................................................................................................. 4
3.2. Write single register (function code 06)....................................................................................................................................... 4
3.2.1. Frame layout .................................................................................................................................................................................. 4
4. Internal registers .............................................................................................................................................................................. 5
4.1. Interface and counter types ........................................................................................................................................................ 6
4.2. General reading registers ........................................................................................................................................................... 6
4.3. Writing registers ......................................................................................................................................................................... 7
4.4. Reading quantities...................................................................................................................................................................... 7
4.4.1. Quantities coded as Integer values ................................................................................................................................................. 9
5. References ...................................................................................................................................................................................... 9
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Modbus Protocol –Technical description
2. Preface
2.1. Overview
The present guide describes the implementation of the Mobus Protocol on the Modbus communication interface.
The interface can be connected to a RS485 Modbus network and can be controlled by a Mobus master station over this network.
Master Rs485
Rs485-Modbus Modbus Counter
Station
Network module
IR interface
The interface supports both the RTU and the ASCII communication protocols modes and it is configurable for what concern to the
transmission parameters (speed, parity, stop bits).
Every parameter is controlled by a Modbus register so you can configure the interface by a sequence of writings into the internal interface
registers.
2.2. Default settings
These are the factory default settings:
• Protocol: Modbus RTU
• Modbus Address: 001
• Baud rate: 19200 bit/s
• Parity: None
• Stop bits: 1
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Modbus Protocol –Technical description
3. Modbus commands
The interface supports only two type of commands, one for reading the register values, one for writing the configuration registers. The
reading is only possible for a block of registers (the command for a single register reading is not supported).
3.1. Read holding registers (function code 03)
This function code is used to read the contents of a contiguous block of holding registers in a remote device. The Request frame specifies
the starting register address and the number of registers.
The register data in the response message are packed as two bytes per register, with the binary contents left justified within each byte. For
each register, the first byte contains the low order bits and the second contains the high order bits; note that the interface BE (Basic and Full)
configured with register 4117=0 (value format 32 bit floating point) is an exception to this rule: for each register, the first byte contains the
high order bits and the second contains the low order bits. In many cases more than one register is used to represent the same quantity to
allow a correct representation of the quantity value (for example, the active energy is represented using 4 registers – 8 bytes -) Please refer
to the chapter “Internal registers” for details.
Warning!
Because of the limited size of a Modbus frame, not all the internal registers can be sent on a single reading request. This means
that a complete snapshot can only be acquired performing more (three) read holding registers calls with different starting address.
Example:
poll nr. 1 start 4099 nr. of registers 100
poll nr. 2 start 4197 nr. of registers 100
poll nr. 3 start 4297 nr. of registers 10
3.1.1. Frame layout
ADR 03 STh STl NRh NRl CRCh CRCl
ADR Modbus Address
03 Read holding register function code (fixed)
STh Starting address register (high order bits)
STl Starting address register (low order bits)
NRh Number of registers (high order bits)
NRl Number of registers (low order bits)
CRCh Modbus Checksum (high order bits)
CRCl Modbus Checksum (low order bits)
3.2. Write single register (function code 06)
This function code is used to write a single holding register in a remote device. The Request specifies the address of the register to be
written. The normal response is an echo of the request, returned after the register contents have been written.
3.2.1. Frame layout
ADR 06 RAh RAl RVh RVl CRCh CRCl
ADR Modbus Address
06 Write single register function code (fixed)
RAh Register address (high order bits)
RAl Register address (low order bits)
RVh Register value (high order bits)
RVl Register value (low order bits)
CRCh Modbus Checksum (high order bits)
CRCl Modbus Checksum (low order bits)
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Modbus Protocol –Technical description
4. Internal registers
This is the complete list of the internal registers.
Register Interface & counter types
Address Designation DRM-**-3P DRM-**-1P SA SE Notes
4099 Device type x x x x
4100 Firmware version x x x x
4101 Range overflow alarm x x x x
4102 Running tariff x x x x
4104 PID (Product Identification) bytes 1 and 2 x x x x General
4105 PID – bytes 3 and 4 x x x x reading
4106 PID – bytes 5 and 6 x x x x registers
4107 PID – bytes 7 and 8 x x x x
4108 PID – bytes 9 and 10 x x x x
4109 PID – bytes 11 and 12 x x x x
4110 PID – bytes 13 and 14 x x x x
4111 Protocol type x x x x
4112 Speed x x x x
4113 Parity x x x x
4114 Stop bits x x x x Writing
4115 Modbus address x x x x registers
4116 Reset interface command x x x x
4117 Value format x x x x
4118 Reset energy counters command x x x x
4119 Active Energy 1st phase T1, imp (kWh) x x x x
4123 Active Energy 2nd phase T1, imp (kWh) x x
4127 Active Energy 3rd phase T1, imp (kWh) x x
4131 Active Energy Σ T1, imp (kWh) x x
4135 Active Energy 1st phase T2, imp (kWh) x x x x
4139 Active Energy 2nd phase T2, imp (kWh) x x
4143 Active Energy 3rd phase T2, imp (kWh) x x
4147 Active Energy Σ T2, imp (kWh) x x
4151 Active Power 1st phase (kW) x x x x
4153 Active Power 2nd phase (kW) x x
4155 Active Power 3rd phase (kW) x x
4157 Active Power Σ (kW) x x
4161 Active Energy 1st phase T1, exp (kWh) x x x x
4165 Active Energy 2nd phase T1, exp (kWh) x x
Reading
4169 Active Energy 3rd phase T1, exp (kWh) x x
quantities
4173 Active Energy Σ T1, exp (kWh) x x registers
4177 Active Energy 1st phase T2, exp (kWh) x x x x
4181 Active Energy 2nd phase T2, exp (kWh) x x
4185 Active Energy 3rd phase T2, exp (kWh) x x
4189 Active Energy Σ T2, exp (kWh) x x
4193 Reactive Energy 1st phase T1, imp (kvarh) x x x x
4197 Reactive Energy 2nd phase T1, imp (kvarh) x x
4201 Reactive Energy 3rd phase T1, imp (kvarh) x x
4205 Reactive Energy Σ T1, imp (kvarh) x x
4209 Reactive Energy 1st phase T2, imp (kvarh) x x x x
4213 Reactive Energy 2nd phase T2, imp (kvarh) x x
4217 Reactive Energy 3rd phase T2, imp (kvarh) x x
4221 Reactive Energy Σ T2, imp (kvarh) x x
4225 Reactive Energy 1st phase T1, exp (kvarh) x x x x
4229 Reactive Energy 2nd phase T1, exp (kvarh) x x
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Modbus Protocol –Technical description
4233 Reactive Energy 3rd phase T1, exp (kvarh) x x
4237 Reactive Energy Σ T1, exp (kvarh) x x
4241 Reactive Energy 1st phase T2, exp (kvarh) x x x x
4245 Reactive Energy 2nd phase T2, exp (kvarh) x x
4249 Reactive Energy 3rd phase T2, exp (kvarh) x x
4253 Reactive Energy Σ T2, exp (kvarh) x x
4257 Reactive Power 1st phase (kvar) x x x x
4259 Reactive Power 2nd phase (kvar) x x
4261 Reactive Power 3rd phase (kvar) x x
4263 Reactive Power Σ (kvar) x x
4267 L1-N voltage (V) x x
4269 L2-N voltage (V) x
4271 L3-N voltage (V) x
4273 L1-L2 voltage (V) x
4275 L2-L3 voltage (V) x
4277 L3-L1 voltage (V) x
4279 phase1 current (A) x x
4281 phase2 current (A) x
4283 phase3 current (A) x
4285 apparent power phase1 (kVA) x x
4287 apparent power phase2 (kVA) x
4289 apparent power phase3 (kVA) x
4291 apparent power Σ (kVA) x
4295 power factor cos φ phase1 x x
4297 power factor cos φ phase2 x
4299 power factor cos φ phase3 x
4301 power factor cos φ Σ x
4303 frequency (Hz) x x
4.1. Interface and counter types
Depending on the type of the Modbus interface (Basic or Full) you are working with and depending also on the counter connected to the
Modbus interface, you have a different set of registers at your disposal.
In the table above, you can see four columns where all the possible combination are listed:
DRM-**-3P Three-Phase counter and Full interface. All the quantities.
TE Three-Phase counter and Basic interface. Energy quantities on all the phases.
DRM-**-1P Single-Phase counter and Full interface. All the quantities on a single phase.
SE Single-Phase counter and Basic interface. Energy quantities on a single phase.
Anyway, all the registers can always be read but if you try to access a register not supported in the combination counter-interface you have,
you will get a value of 0.
Example: if you try to read the register 4267 (voltage on L1) when you have a three-phase counter and a Basic interface (TE column) you will
always get a value of 0, because the interface is not allowed to send out this quantity.
4.2. General reading registers
This family of registers store general information about the interface.
All the registers are always available regardless to the combination counter-interface you have.
Register Designation Description
4099 Device type Code that identify the combination interface-counter
0 No communication with the counter on the IR port
1 Three-phase Full
2 Three-phase Basic
3 Single-phase Full
4 Single-phase Basic
4100 Firmware version Version of the interface firmware
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Modbus Protocol –Technical description
4101 Range overflow alarm The register is set by the counter if it has the detected a value over the voltage or the current
nominal threshold.
The lowest order byte of the register is bit-coded as follows:
n.u.|n.u.|OFV3|OFI3|OFV2|OFI2|OFV1|OFI1
Where:
OFV Voltage overflow (on phase 1, 2 and 3)
OFI Current overflow (on phase 1, 2 and 3)
n.u. Not Used
4102-03 Running tariff 0 Tariff 1 is currently in use
1 Tariff 2 is currently in use
4104-10 PID Part number identification string (a maximum of 14 bytes)
4.3. Writing registers
This set of registers is for the interface configuration. One register (4118) is dedicated to request the reset of the counter internal energy
registers.
All the registers are always available regardless to the combination counter-interface you have.
The registers from 4111 to 4115 are controlled by the Reset interface command register (4116): all the changing you make to the first ones
take effect only when you ask a reset of the interface by assigning a value of 1 to the last one.
Any change to the registers 4117 and 4118 is immediately effective.
Register Designation Description
4111 Protocol type 0 Modbus RTU protocol
1 Modbus ASCII protocol
4112 Speed One of the following:
1200, 2400, 4800, 9600, 19200, 38400
4113 Parity 0 None
1 Even
2 Odd
4114 Stop bits 1 or 2
4115 Modbus address From 1 to 247
4116 Reset interface command 0 Changes made on registers 4111-4115 are not effective
1 Changes made on registers 4111-4115 take effect
4117 Value format 0 Quantities coded as floating point 32 bit
1 Quantities coded as integers (see par. 4.4)
4118 Reset energy counters command 1 Reset active energy registers
2 Reset reactive energy registers
3 Reset all the registers
Note!
The register 4118, is a “pass-through” register because the final target of the command is the counter
connected to the interface. If you change the register value, a command will be given to the counter in
order to call a reset of the counter internal registers.
All the other writing registers modify the interface behaviour.
4.4. Reading quantities
These registers holds the electrical quantities controlled by the counter connected to the interface. As stated in paragraph
4.1 the available quantities are dependent from the combination counter/interface type you have (TA: Three phase
counter/Full interface, TE: Three phase counter/Basic interface, SA: Single phase counter/Full interface, SE: Single phase
counter/Basic interface).
Register Designation Interface & counter types Length
address DRM-**-3P TE DRM-**-1P SE (bytes)
4119 Active Energy 1st phase T1, imp (kWh) x x x x 8
4123 Active Energy 2nd phase T1, imp (kWh) x x 8
4127 Active Energy 3rd phase T1, imp (kWh) x x 8
4131 Active Energy Σ T1, imp (kWh) x x 8
4135 Active Energy 1st phase T2, imp (kWh) x x x x 8
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Modbus Protocol –Technical description
4139 Active Energy 2nd phase T2, imp (kWh) x x 8
4143 Active Energy 3rd phase T2, imp (kWh) x x 8
4147 Active Energy Σ T2, imp (kWh) x x 8
4151 Active Power 1st phase (kW) x x x x 4
4153 Active Power 2nd phase (kW) x x 4
4155 Active Power 3rd phase (kW) x x 4
4157 Active Power Σ (kW) x x 8
4161 Active Energy 1st phase T1, exp (kWh) x x x x 8
4165 Active Energy 2nd phase T1, exp (kWh) x x 8
4169 Active Energy 3rd phase T1, exp (kWh) x x 8
4173 Active Energy Σ T1, exp (kWh) x x 8
4177 Active Energy 1st phase T2, exp (kWh) x x x x 8
4181 Active Energy 2nd phase T2, exp (kWh) x x 8
4185 Active Energy 3rd phase T2, exp (kWh) x x 8
4189 Active Energy Σ T2, exp (kWh) x x 8
4193 Reactive Energy 1st phase T1, imp (kvarh) x x x x 8
4197 Reactive Energy 2nd phase T1, imp (kvarh) x x 8
4201 Reactive Energy 3rd phase T1, imp (kvarh) x x 8
4205 Reactive Energy Σ T1, imp (kvarh) x x 8
4209 Reactive Energy 1st phase T2, imp (kvarh) x x x x 8
4213 Reactive Energy 2nd phase T2, imp (kvarh) x x 8
4217 Reactive Energy 3rd phase T2, imp (kvarh) x x 8
4221 Reactive Energy Σ T2, imp (kvarh) x x 8
4225 Reactive Energy 1st phase T1, exp (kvarh) x x x x 8
4229 Reactive Energy 2nd phase T1, exp (kvarh) x x 8
4233 Reactive Energy 3rd phase T1, exp (kvarh) x x 8
4237 Reactive Energy Σ T1, exp (kvarh) x x 8
4241 Reactive Energy 1st phase T2, exp (kvarh) x x x x 8
4245 Reactive Energy 2nd phase T2, exp (kvarh) x x 8
4249 Reactive Energy 3rd phase T2, exp (kvarh) x x 8
4253 Reactive Energy Σ T2, exp (kvarh) x x 8
4257 Reactive Power 1st phase (kvar) x x x x 4
4259 Reactive Power 2nd phase (kvar) x x 4
4261 Reactive Power 3rd phase (kvar) x x 4
4263 Reactive Power Σ (kvar) x x 8
4267 L1-N voltage (V) x x 4
4269 L2-N voltage (V) x 4
4271 L3-N voltage (V) x 4
4273 L1-L2 voltage (V) x 4
4275 L2-L3 voltage (V) x 4
4277 L3-L1 voltage (V) x 4
4279 phase1 current (A) x x 4
4281 phase2 current (A) x 4
4283 phase3 current (A) x 4
4285 apparent power phase1 (kVA) x x 4
4287 apparent power phase2 (kVA) x 4
4289 apparent power phase3 (kVA) x 4
4291 apparent power total (kVA) x 8
4295 power factor cos phi phase1 x x 4
4297 power factor cos phi phase2 x 4
4299 power factor cos phi phase3 x 4
4301 power factor cos phi total x 4
4303 frequency (Hz) x x 4
Notes
T1/T2 stand for Tariff 1 and tariff 2.
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Modbus Protocol –Technical description
The symbol Σ indicates a total amount (for example: the Reactive Power Σ (kvar) value is the total Reactive Power on the three phases. It is
of course significant only if you have a three phase counter connected to the interface).
imp/exp (imported/exported) indicates whether the energy is generated (exported) or consumpted (imported).
Length in bytes of the quantity. Note that because a Modbus register is 2 bytes long, all the quantities are splitted on more registers (4
bytes: 2 registers; 8 bytes: 4 registers).
Tip!
Remember that all the quantities are coded as 32 bit floating point values.
Each type of interface (Basic or Full) is available in two versions:
BE – the floating point values are transmitted in big-endian format
LE – the floating point values are transmitted in little-endian format
If you want to switch to an integer representation, you have to change the value of the configuration register 4117 (see the
paragraph 4.3)
4.4.1. Quantities coded as Integer values
While the notation using floating point 32 bit values is unambigous, when you switch to the integer notation something must be explained in
order to allow the correct interpretation of original value.
Quantities 4 bytes long
The integer value stored in these registers (2) must be devided by a factor of 10000 to rebuild the original value.
Example: Active Power 1st phase
Integer value: 122447
Original value: 122447/10000=12,2447 (kW)
Quantities 8 bytes long
The rebuilding of the original value is slightly more complicated.
The value stored in the first 4 bytes must be multiplied by a factor of 10^9 (1000000000).
Then it must be added to the value stored in the following 4 bytes.
Finally, the result must be divided by 10000.
Example: Active Power total
Integer value (most significat 4 bytes): 12344
Integer value (less significant 4 bytes): 765532
Original value: (12344*1000000000+765532)/10000=1234400076,5532 (kW)
5. References
For any further information concerning the Modbus protocol implementation, you can consult the following documents and references:
Modbus application protocol specifications V 1.1b, at http://www.modbus-IDA.org
Modbus over serial line – Specification and implementation guide V. 1.02, at http://www.modbus.org
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Modbus Protocol –Technical description
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