Lumel NMID30-2 User manual

NMID30-2
1 and 3-phase energy meter 100 A
(MID certied)
User’s manual - MODBUS

ModBus Protocol Implementation 1
1. NMID30-2 Protocol Implementation
1.1 Modbus Protocol Overview
1.2 Input register
1.2.1 NMID30-2 Input Registers
1.3 Modbus Protocol Holding Registers and Digital meter set up:
1.3.1 MODBUS Protocol Holding Register Parameters
2 RS485 General Information
2.1 Half Duplex
2.2 Connecting the Instruments
2.3 A and B terminals
2.4 Troubleshooting
3 MODBUS Protocol General Information
3.1 MODBUS Protocol Message Format
3.2 Serial Transmission Modes
3.3 MODBUS Protocol Message Timing (RTU Mode)
3.4 How Characters are Transmitted Serially
3.5 Error Checking Methods
3.5.1 Parity Checking
3.5.2 CRC Checking
3.6 Function Codes
3.7 IEEE floating point format
3.8 MODBUS Protocol Commands supported
3.8.1 Read Input Registers
3.9 Holding Registers
3.9.1 Read Holding Registers
3.9.2 Write Holding Registers
3.10 Exception Response
3.11 Exception Codes
3.11.1 Table of Exception Codes
3.12 Diagnostics
NMID30-2 _ Modbus

ModBus Protocol Implementation 2
1. NMID30-2 Smart Meter Modbus Protocol Implementation
1.1 Modbus Protocol Overview
This section provides basic information for interfacing the Smart meter to a Modbus Protocol network. If background information or more details of
the Smart implementation is required please refer to section 2 and 3 of this document.
RS485 is a bidirectional, full or half-duplex communication bus structure consisting of a single Master and at least one Slave. The maximum number
of slaves can vary widely from system to system however; most manufacturers cap the max number of slaves between 16 and 32. Most RS485
signals operate ideally on a DC bias of 5 volts. The signals are driven alternately from each other. That is, each line operates inversely from the other
and each is also referenced to the other from an electrical standpoint. The receiver looks at the difference, not the absolute voltage value, of the two
signals. This is referred to as the “Line Bias” and it is critical in RS-485 applications. A bias difference of higher than 0.3 v is generally accepted as
valid but can be as high as 0.7v, depending on the system. Absolute values below this are considered “undefined” or “grey” and may result in either
a high or low reading by the receiver. In many nonisolated applications, a “signal ground” or “shield” connection is provided in addition to the 2 data
lines however, this is not necessary as the signals are referenced to each other and not to absolute ground. It is only provided as a ground point for
the communication cable shielding. It is important to note that the shield ground should only be connected at a single trunk end in non-isolated
applications. If grounds are tied together in non-isolated systems, even a slight voltage difference, between the absolute ground points, will create a
“ground loop” condition which can cause serious damage to the equipment. Note that in isolated equipment, the above does not apply as there is
no direct connection between the ground points however, in all applications, proper cable shield grounding should be practiced to eliminate and/or
reduce electrical interference.
Our NMID30-2 offers the option of an RS485 communication facility for direct connection to SCADA or other communications systems using the
Modbus Protocol RTU salve protocol. The Modbus Protocol establishes the format for the master’s query by placing into it the device address, a
function code defining the requested action, any data to be sent, and an error checking field. The slave’s response message is also constructed
using Modbus Protocol. It contains fields confirming the action taken, any data to be returned, and an error-checking field. If an error occurs in
receipt of the message, the NMID30-2 will make no response. If the NMID30-2 is unable to perform the requested action, it will construct an error
message and send it as the response.
The electrical interface is 2-wire RS485, via 2 screw terminals. Connection should be made using twisted pair screened cable (Typically 22 gauge
Belden 8761 or equivalent). All “A” and “B” connections are daisy chained together. Line topology may or may not require terminating loads
depending on the type and length of cable used. Loop (ring) topology does not require any termination load. The impedance of the termination load
should match the impedance of the cable and be at both ends of the line. The cable should be terminated at each end with a 120 ohm (0.25 Watt
min.) resistor. A total maximum length of 3900 feet (1200 meters) is allowed for the RS485 network. A maximum of 32 electrical nodes can be
connected, including the controller. The address of each the NMID30-2 can be set to any value between 1 and 247. Broadcast mode (address 0) is
not supported.
The format for each byte in RTU mode is:
Coding System: 8-bit per byte
Data Format: 4 bytes (2 registers) per parameter.
Floating point format ( to IEEE 754)
Most significant register first (Default). The default may be changed if required -See Holding
Register “Register Order” parameter.
Error Check Field: 2 byte Cyclical Redundancy Check (CRC)
Framing: 1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity (or no parity)
1 stop bit if parity is used; 1 or 2 bits if no parity
Data Coding
All data values in the NMID30-2 meter are transferred as 32 bit IEEE754 floating point numbers, (input and output) therefore each NMID30-2 meter
value is transferred using two Modbus Protocol registers. All register read requests and data write requests must specify an even number of
registers. Attempts to read/write an odd number of registers prompt the NMID30-2 to return a Modbus Protocol exception message. However, for
compatibility with some SCADA systems, the NMID30-2 will respond to any single input or holding register read with an instrument type specific
value. The NMID30-2 can transfer a maximum of 40 values in a single transaction; therefore the maximum number of registers requestable is 80.
Exceeding this limit prompts the NMID30-2 to generate an exception response.
Data transmission speed is selectable between 2400, 4800, 9600, 19200, 38400 baud.
1.2 Input register
Input registers are used to indicate the present values of the measured and calculated electrical quantities. Each parameter is held in two
consecutive16 bit register. The following table details the 3X register address, and the values of the address bytes within the message. A ( ) in the
column indicates that the parameter is valid for the particular wiring system. Any parameter with a cross (X) will return the value zero. Each parameter
is held in the 3X registers. Modbus Protocol function code 04 is used to access all parameters.
For example, to request: Amps 1 Start address =0006
No. of registers =0002
Amps 2 Start address =0008
No. of registers =0002
Each request for data must be restricted to 40 parameters or less. Exceeding the 40 parameter limit will cause a Modbus Protocol exception code to
be returned.
NMID30-2 _ Modbus
√

ModBus Protocol Implementation 3
1.2.1 NMID30-2 Input Registers
Address Parameter NMID30-2 Input Registers Input Register Modbus 3Ø 3Ø 1Ø
(Register) Number Parameter Protocol
Start
Address
Hex
Description Units Hi Lo 4W 3W 2W
Byte Byte
30001 1 Phase 1 line to neutral volts. Volts 00 00 √ X √
30003 2 Phase 2 line to neutral volts. Volts 00 02 √ X X
30005 3 Phase 3 line to neutral volts. Volts 00 04 √ X X
30007 4 Phase 1 current. Amps 00 06 √ √ √
30009 5 Phase 2 current. Amps 00 08 √ √ X
30011 6 Phase 3 current. Amps 00 0A √ √ X
30013 7 Phase 1 power. Watts 00 0C √ X √
30015 8 Phase 2 power. Watts 00 0E √ X √
30017 9 Phase 3 power. Watts 00 10 √ X X
30019 10 Phase 1 volt amps. VoltAmps 00 12 √ X √
30021 11 Phase 2 volt amps. VoltAmps 00 14 √ X X
30023 12 Phase 3 volt amps. VoltAmps 00 16 √ X X
30025 13 Phase 1 volt amps reactive. VAr 00 18 √ X √
30027 14 Phase 2 volt amps reactive. VAr 00 1A √ X X
30029 15 Phase 3 volt amps reactive. VAr 00 1C √ X X
30031 16 Phase 1 power factor (1). None 00 1E √ X √
30033 17 Phase 2 power factor (1). None 00 20 √ X X
30035 18 Phase 3 power factor (1). None 00 22 √ X X
30037 19 Phase 1 phase angle. Degrees 00 24 √ X √
30039 20 Phase 2 phase angle. Degrees 00 26 √ X X
30041 21 Phase 3 phase angle. Degrees 00 28 √ X X
30043 22 Average line to neutral volts. Volts 00 2A √ X X
30047 24 Average line current. Amps 00 2E √ √ √
30049 25 Sum of line currents. Amps 00 30 √ √ √
30053 27 Total system power. Watts 00 34 √ √ √
30057 29 Total system volt amps. VA 00 38 √ √ √
30061 31 Total system VAr. VAr 00 3C √ √ √
30063 32 Total system power factor (1). None 00 3E √ √ √
30067 34 Total system phase angle. Degrees 00 42 √ √ √
30071 36 Frequency of supply voltages. Hz 00 46 √ √ √
30073 37 Import Wh since last reset (2). kWh/MWh 00 48 √ √ √
30075 38 Export Wh since last reset (2). kWh/MWh 00 4A √ √ √
30077 39 Import VArh since last reset (2). kVArh/MVArh 00 4C √ √ √
30079 40 Export VArh since last reset (2). kVArh/MVArh 00 4E √ √ √
30081 41 VAh since last reset (2). kVAh/MVAh 00 50 √ √ √
30083 42 Ah since last reset (3). Ah/kAh 00 52 √ √ √
30085 43 Total system power demand (4). Watts 00 54 √ √ √
30087 44 Maximum total system power demand (4). Watts 00 56 √ √ √
30101 51 Total system VA demand. VA 00 64 √ √ √
30103 52 Maximum total system VA demand. VA 00 66 √ √ √
30105 53 Neutral current demand. Amps 00 68 √ X X
30107 54 Maximum neutral current demand. Amps 00 6A √ X X
30201 101 Line 1 to Line 2 volts. Volts 00 C8 √ √ X
30203 102 Line 2 to Line 3 volts. Volts 00 CA √ √ X
30205 103 Line 3 to Line 1 volts. Volts 00 CC √ √ X
30207 104 Average line to line volts. Volts 00 CE √ √ X
NMID30-2 _ Modbus

ModBus Protocol Implementation 4
30225 113 Neutral current. Amps 00 E0 √ X X
30235 118 Phase 1 L/N volts THD % 00 EA √ X √
30237 119 Phase 2 L/N volts THD % 00 EC √ X X
30239 120 Phase 3 L/N volts THD % 00 EE √ X X
30241 121 Phase 1 Current THD % 00 F0 √ √ √
30243 122 Phase 2 Current THD % 00 F2 √ √ X
30245 123 Phase 3 Current THD % 00 F4 √ √ X
30249 125 Average line to neutral volts THD. % 00 F8 √ X √
30251 126 Average line current THD. % 00 FA √ √ √
30255 128 -Total system power factor (5). Degrees 00 FE √ √ √
30259 130 Phase 1 current demand. Amps 01 02 √ √ √
30261 131 Phase 2 current demand. Amps 01 04 √ √ X
30263 132 Phase 3 current demand. Amps 01 06 √ √ X
30265 133 Maximum phase 1 current demand. Amps 01 08 √ √ √
30267 134 Maximum phase 2 current demand. Amps 01 0A √ √ X
30269 135 Maximum phase 3 current demand. Amps 01 0C √ √ X
30335 168 Line 1 to line 2 volts THD. % 01 4E √ √ X
30337 169 Line 2 to line 3 volts THD. % 01 50 √ √ X
30339 170 Line 3 to line 1 volts THD. % 01 52 √ √ X
30341 171 Average line to line volts THD. % 01 54 √ √ X
30343 172 Total active energy kWh 01 56 √ √ √
30345 173 Total reactive energy kVarh 01 58 √ √ √
30347 174 Phase 1 import active energy kWh 01 5a √ √ √
30349 175 Phase 2 import active energy kWh 01 5c √ √ √
30351 176 Phase 3 import active energy kWh 01 5e √ √ √
30353 177 Phase 1 export active energy kWh 01 60 √ √ √
30355 178 Phase 2 export active energy kWh 01 62 √ √ √
30357 179 Phase 3 export active energy kWh 01 64 √ √ √
30359 180 Phase 1 total active energy kWh 01 66 √ √ √
30361 181 Phase 2 total active energy kWh 01 68 √ √ √
30363 182 Phase 3 total active energy kWh 01 6a √ √ √
30365 183 Phase 1 import reactive energy kVArh 01 6c √ √ √
30367 184 Phase 2 import reactive energy kVArh 01 6e √ √ √
30369 185 Phase 3 import reactive energy kVArh 01 70 √ √ √
30371 186 Phase 1 export reactive energy kVArh 01 72 √ √ √
30373 187 Phase 2 export reactive energy kVArh 01 74 √ √ √
30375 188 Phase 3 export reactive energy kVArh 01 76 √ √ √
30377 189 Phase 1 total reactive energy kVArh 01 78 √ √ √
30379 190 Phase 2 total reactive energy kVArh 01 7a √ √ √
30381 191 Phase 3 total reactive energy kVArh 01 7c √ √ √
Notes:
1. The power factor has its sign adjusted to indicate the nature of the load. Positive for capacitive and negative for inductive.
2. There is a user option to select either k or M for the energy prefix.
3. The same user option as in 2 above gives a prefix of none or k for Amp hours
4. The power sum demand calculation is for import power only
5. The negative total system power factor is a sign inverted version of parameter 32, the magnitude is the same as parameter 32.
6. There is a user option to select None, k or M for the energy prefix.
1.3 Modbus Protocol Holding Registers and Digital meter set up:
Holding registers are used to store and display instrument configuration settings. All holding registers not listed in the table below should be
considered as reserved for manufacturer use and no attempt should be made to modify their values.
The holding register parameters may be viewed or changed using the Modbus Protocol. Each parameter is held in two consecutive 4X registers.
Modbus Protocol Function Code 03 is used to read the parameter and Function Code 16 is used to write. Write to only one parameter per message.
NMID30-2 _ Modbus

ModBus Protocol Implementation 5
1.3.1 MODBUS Protocol Holding Register Parameters
Address Parameter Parameter Modbus Valid range Mode
Register Number Protocol
Start Address
Hex
High Low
Byte Byte
40001 1 Demand Time 00 00 Read minutes into first demand calculation. When the Demand Time Ro
reaches the Demand Period then the demand values are valid.
40003 2 Demand Period 00 02 Write d emand period: 0, 5,8, 10, 15, 20, 30 or 60 minutes, default 60. r/w
Setting the period to 0 will cause the demand to show the current
parameter value, and demand max to show the maximum parameter
value since last demand reset.
40007 4 System Volts 00 06 Read system voltage, VLL for 3P3W, VLN for others. ro
40009 5 System Current 00 06 Write system current, limited to 1 to 9999A.Requires password, see ro
parameter 13
40011 6 System Type 00 08 Write system type: 3p4w = 3, 3p3w = 2 & 1p2w= 1 r/wp
Requires password, see parameter 13
40013 7 Relay Pulse Width 00 OA Write relay on period in milliseconds: 60, 100 or 200, default 200. r/wp
40015 8 Password Lock 00 OE Write any value to password lock protected registers. r/w
Read password lock status:
0 = locked. 1 = unlocked.
Reading will also reset the password timeout back to one minute.
40019 10 Network Parity Stop 00 12 Write the network port parity/stop bits for MODBUS Protocol, where: r/w
0 = One stop bit and no parity, default. 1 = One stop bit and even
parity. 2 = One stop bit and odd parity.3 = Two stop bits and no parity.
Requires a restart to become effective.
40021 11 Network Node 00 14 Write the network port node address: 1 to 247 for MODBUS r/w
Protocol, default 1. Requires a restart to become effective.
Note, both the MODBUS Protocol and Johnson Controls node
addresses can be changed via the display setup menus.
40023 12 Pulse Divisor 00 16 Write pulse divisor index: n = 2 to 6 in Wh/l0^n, default 3. r/w
40025 13 Password 00 18 Write password for access to protected registers. Read zero. Reading r/w
will also reset the password timeout back to one minute. Default
password is 0000.
40029 15 Network Baud Rate 00 1C Write the network port baud rate for MODBUS Protocol, where: r/w
0 = 2400 baud. 1 = 4800 baud.
2 = 9600 baud, default.
3 = 19200 baud. 4 = 38400 baud. Requires a restart to become
effective
40031 16 Energy Units Prefix 00 1E Write the units prefix for energy output values. 0 = k, e.g. kWh, r/w
default. But Ah for ampere hours. 1 = M, e.g. MWh. But kAh for
ampere hours.
40037 19 System Power 00 24 Read the total system power, e.g. for 3p4w returns System Volts x ro
System Amps x 3.
Password
Some of the parameters described above are password protected and thus require the password to be entered at the Password register before they can be changed.
The default password is 1000. When the password has been entered it will timeout in one minute unless the Password or Password Lock register is read to reset the
timeout timer. Once the required changes have been made to the protected parameters the password lock should be reapplied by
a) Allowing the password to timeout, or
b) Writing any value to the Password Lock register, or
c) Power cycling the instrument.
NMID30-2 _ Modbus
40087 44 Pulse 1 Energy Type 00 56 Write MODBUS Protocol input parameter for pulse r/w
output 1:
40089 45 Pulse 2 Energy Type 00 58 MODBUS Protocol input parameter for pulse output r/w
461457 3072 Reset F0 10 Reset of Demand maximum values. Value type: Hex (16 bit)
4: exported active energy, 5: imported reactive energy
6: total reactive energy, 8: exported reactive energy
Value type: float (32 bits)
1: imported active energy , 2: total active energy
1: imported active energy , 2: total active energy
4: exported active energy, 5: imported reactive energy
6: total reactive energy, 8: exported reactive energy
Value type: float (32 bits)
wo
464513 464513 Serial number FC 00 Serial number
Value type: unsigned int (32 bit)
Read-only registry
ro
Register Order NMID30-1 receives or sends floating-point numbers:
It is perfectly feasible to change the NMID30-1 set-up using a general purpose Modbus Protocol master, but often easier to use the NMID30-1 display, especially for gaining
password protected access.
in 3210 byte order - the oldest is sent first.

ModBus Protocol Implementation 6
The Following information in this section relates to the NMID30-2 and is included to assist where a mixed network is implemented. RS485 or EIA
(Electronic Industries Association) RS485 is a balanced line, half-duplex transmission system allowing transmission distances of up to 1.2 km. The
following table summarizes the RS-485 Standard:
PARAMETER
Mode of Operation Differential
Number of Drivers and Receivers 32 Drivers, 32 Receivers
Maximum Cable Length 1200 m
Maximum Data Rate 10 M baud
Maximum Common Mode Voltage 12 V to –7 V
Minimum Driver Output Levels (Loaded) +/– 1.5 V
Minimum Driver Output Levels (Unloaded) +/– 6 V
Drive Load Minimum 60 ohms
Driver Output Short Circuit Current Limit 150 mA to Gnd,
250 mA to 12 V
250 mA to –7 V
Minimum Receiver Input Resistance 12 kohms
Receiver Sensitivity +/– 200 mV
Further information relating to RS485 may be obtained from either the EIA or the various RS485 device manufacturers, for example Texas
Instruments or Maxim Semiconductors. This list is not exhaustive.
2.1 Half Duplex
Half duplex is a system in which one or more transmitters (talkers) can communicate with one or more receivers (listeners) with only one transmitter
being active at any one time. For example, a “conversation” is started by asking a question, the person who has asked the question will then listen
until he gets an answer or until he decides that the individual who was asked the question is not going to reply.
In a 485 network the “master” will start the “conversation” with a “query” addressed to a specific “slave”, the “master” will then listen for the “slave’s”
response. If the “slave” does not respond within a pre-defined period, (set by control software in the “master”), the “master” will abandon the
“conversation”.
2.2 Connecting the Instruments
If connecting an RS485 network to a PC use caution if contemplating the use of an RS232 to 485 converters together with a USB to RS485 adapter.
Consider either an RS232 to RS485 converter, connected directly to a suitable RS232 jack on the PC, or use a USB to RS485 converter or, for
desktop PCs a suitable plug in RS485 card. (Many 232:485 converters draw power from the RS232 socket. If using a USB to RS232 adapter, the
adapter may not have enough power available to run the 232:485 converter.)
Screened twisted pair cable should be used. For longer cable runs or noisier environments, use of a cable specifically designed for RS485 may be
necessary to achieve optimum performance. All “A” terminals should be connected together using one conductor of the twisted pair cable, all “B”
terminals should be connected together using the other conductor in the pair.
A Belden 9841 (Single pair) or 9842 (Two pair) or similar cable with a characteristic impedance of 120 ohms is recommended. The cable should be
terminated at each end with a 120 ohm, wquarter watt (or greater) resistor. Note: Diagram shows wiring topology only. Always follow terminal
identification on the user manual.
There must be no more than two wires connected to each terminal, this ensures that a “Daisy Chain or “straight line” configuration is used. A “Star”
or a network with “Stubs (Tees)” is not recommended as reflections within the cable may result in data corruption.
NMID30-2 _ Modbus

ModBus Protocol Implementation 7
2.3 A and B terminals
The A and B connections to the NMID30-2 Digital meter products can be identified by the signals present on them whilst there is activity on the
RS485 bus:
2.4 Troubleshooting
 Start with a simple network, one master and one slave. With the NMID30-2 Digital meter this is easily achieved as the network can be left intact
whilst individual instruments are disconnected by removing the RS485 connection from the rear of the instrument.
 Check that the network is connected together correctly. That is all of the “A’s” are connected together, and all of the “B’s” are connected
together. Confirm that the data “transmitted” onto the RS485 is not echoed back to the PC on the RS232 lines. (This facility is sometimes a link
option within the converter). Many PC based packages seem to not perform well when they receive an echo of the message they are
transmitting. SpecView and PCView (PC software) with a RS232 to RS485 converter are believed to include this feature.
 Confirm that the Address of the instrument is the same as the “master” is expecting.
 If the “network” operates with one instrument but not more than one check that each instrument has a unique address.
 Each request for data must be restricted to 40 parameters or less. Violating this requirement will impact the performance of the instrument and
may result in a response time in excess of the specification.
 Check that the MODBUS Protocol mode (RTU or ASCII) and serial parameters (baud rate, number of data bits, number of stop bits and parity)
are the same for all devices on the network.
 Check that the “master” is requesting floating-point variables (pairs of registers placed on floating point boundaries) and is not “splitting” floating
point variables.
 Check that the floating-point byte order expected by the “master” is the same as that used by the NMID30-2. (PCView and Citect packages can
use a number of formats including that supported by this meter).
 If possible obtain a second RS232 to RS485 converter and connect it between the RS485 bus and an additional PC equipped with a software
package, which can display the data on the bus. Check for the existence of valid requests.
Communication on a MODBUS Protocol Network is initiated (started) by a “Master” sending a query to a “Slave”. The “Slave“, which is constantly
monitoring the network for queries addressed to it, will respond by performing the requested action and sending a response back to the ”Master”.
Only the “Master” can initiate a query.
In the MODBUS Protocol the master can address individual slaves, or, using a special “Broadcast” address, can initiate a broadcast message to all
slaves. The NMID30-2 Digital meter do not support the broadcast address.
NMID30-2 _ Modbus

ModBus Protocol Implementation 8
3.1 MODBUS Protocol Message Format
The query contains the device (or broadcast) address, a function code defining the requested action, any data to be sent, and an error-checking
field.
The response contains fields confirming the action taken, any data to be returned, and an error-checking field. If an error occurred in receipt of the
message then the message is ignored, if the slave is unable to perform the requested action, then it will construct an error message and send it as
its response. The MODBUS Protocol functions used by the NMID30-2 Digital meters copy 16 bit register values between master and slaves.
However, the data used by the NMID30-2 Digital meter is in 32 bit IEEE 754 floating point format. Thus each instrument parameter is conceptually
held in two adjacent MODBUS Protocol registers. Query
The following example illustrates a request for a single floating point parameter i.e. two 16-bit Modbus Protocol Registers.
First Byte Last Byte
Slave Function Start Address Start Address Number of Points Number of Points Error Check Error Check
Address Code (Hi) (Lo) (Hi) (Lo) (Lo) (Hi)
Slave Address: 8-bit value representing the slave being addressed (1 to 247), 0 is reserved for the broadcast address. The Digital meters do not
support the broadcast address.
Function Code: 8-bit value telling the addressed slave what action is to be performed. (3, 4, 8 or 16 are valid for Digital meter)
Start Address (Hi): The top (most significant) eight bits of a 16-bit number specifying the start address of the data being requested.
Start Address (Lo): The bottom (least significant) eight bits of a 16-bit number specifying the start address of the data being requested. As registers
are used in pairs and start at zero, then this must be an even number.
Number of Points (Hi): The top (most significant) eight bits of a 16-bit number specifying the number of registers being requested.
Number of Points (Lo): The bottom (least significant) eight bits of a 16-bit number specifying the number of registers being requested. As registers
are used in pairs, then this must be an even number.
Error Check (Lo): The bottom (least significant) eight bits of a 16-bit number representing the error check value.
Error Check (Hi): The top (most significant) eight bits of a 16-bit number representing the error check value.
NMID30-2 _ Modbus

ModBus Protocol Implementation 9
Response
The example illustrates the normal response to a request for a single floating point parameter i.e. two 16-bit Modbus Protocol Registers.
Slave Function Byte First Register First Register Second Register Second Register Error Check Error Check
Address Code Count (Hi) (Lo) (Hi) (Lo) (Lo) (HI)
Slave Address: 8-bit value representing the address of slave that is responding.
Function Code: 8-bit value which, when a copy of the function code in the query, indicates that the slave recognized the query and has responded. (See also Exception
Response).
Byte Count: 8-bit value indicating the number of data bytes contained within this response
First Register (Hi)*: The top (most significant) eight bits of a 16-bit number representing the first register requested in the query.
First Register (Lo)*: The bottom (least significant) eight bits of a 16-bit number representing the first register requested in the query.
Second Register (Hi)*: The top (most significant) eight bits of a 16-bit number representing the second register requested in the query.
Second Register (Lo)*: The bottom (least significant) eight bits of a 16-bit number representing the second register requested in the query.
Error Check (Lo): The bottom (least significant) eight bits of a 16-bit number representing the error check value.
Error Check (Hi): The top (most significant) eight bits of a 16-bit number representing the error check value.
*These four bytes together give the value of the floating point parameter requested.
Exception Response
If an error is detected in the content of the query (excluding parity errors and Error Check mismatch), then an error response (called an exception response), will be sent to
the master. The exception response is identified by the function code being a copy of the query function code but with the most -significant bit set. The data contained in
an exception response is a single byte error code.
First Byte Last Byte
Slave Function Error Error Check Error Check
Address Code Code (Lo) (Hi)
Slave Address: 8-bit value representing the address of slave that is responding.
Function Code: 8 bit value which is the function code in the query OR’ed with 80 hex, indicating that the slave either does not recognize the query or could not carry out
the action Requested.
Error Code: 8-bit value indicating the nature of the exception detected. (See “Table Of Exception Codes“ later).
Error Check (Lo): The bottom (least significant) eight bits of a 16-bit number representing the error check value.
Error Check (Hi): The top (most significant) eight bits of a 16-bit number representing the error check value.
3.2 Serial Transmission Modes
There are two MODBUS Protocol serial transmission modes, ASCII and RTU. The NMID30-2 Digital meter do not support the ASCII mode.
In RTU (Remote Terminal Unit) mode, each 8-bit byte is used in the full binary range and is not limited to ASCII characters as in ASCII Mode. The
greater data density allows better data throughput for the same baud rate, however each message must be transmitted in a continuous stream. This
is very unlikely to be a problem for modern communications equipment.
Line Protocol: 1 start bit, followed by the 8 data bits. The 8 data bits are sent with least significant bit first.
User Option Of Parity No Parity and 2 Stop Bits
And Stop Bits: No Parity and 1 Stop Bit
Even Parity and 1 Stop Bit
Odd Parity and 1 Stop Bit.
User Option of Baud 4800 ; 9600 ; 19200 ; 38400
Rate: Digital meters do not support 38400 but do offer 2400 instead)
The baud rate, parity and stop bits must be selected to match the master’s settings.
3.3 MODBUS Protocol Message Timing (RTU Mode)
A MODBUS Protocol message has defined beginning and ending points. The receiving devices recognizes the start of the message, reads the
“Slave Address” to determine if they are being addressed and knowing when the message is completed they can use the Error Check bytes and
parity bits to confirm the integrity of the message. If the Error Check or parity fails then the message is discarded.
In RTU mode, messages starts with a silent interval of at least 3.5 character times.
The first byte of a message is then transmitted, the device address.
Master and slave devices monitor the network continuously, including during the ‘silent’ intervals. When the first byte (the address byte) is received, each device checks it
to find out if it is the addressed device. If the device determines that it is the one being addressed it records the whole message and acts accordingly, if it is not being
addressed it continues monitoring for the next message.
Following the last transmitted byte, a silent interval of at least 3.5 character times marks the end of the message. A new message can begin after this interval. In the
NMID30-2 1000 and 2000, a silent interval of 60msec minimum is required in order to guarantee successful reception of the next request.
The entire message must be transmitted as a continuous stream. If a silent interval of more than 1.5 character times occurs before completion of the message, the
receiving device flushes the incomplete message and assumes that the next byte will be the address byte of a new message.
Similarly, if a new message begins earlier than 3.5 character times following a previous message, the receiving device may consider it a continuation of the previous
message. This will result in an error, as the value in the final CRC field will not be valid for the combined messages.
NMID30-2 _ Modbus