High Accuracy Digital Power Meter
Added with Extension modules
Suits your various applications
Communication Guide[English]
Rev 1.3
Page 2
Notice
Limitation of Liability
Rootech Inc. reserves the right to make changes in the device specifications shown in
this User’s Guide without notice. Rootech Inc. advises customers to obtain the latest
specifications of device before giving orders to confirm that the information being relied
on by the customer is up to date.
In the absence of written agreement to the contrary, Rootech Inc. assumes no liability
for Rootech Inc. applications assistance, customer’s system design, or infringement of
patents or copyrights of third parties by or arising from the use of devices described
herein.
Except to the extent prohibited by applicable law, under no circumstances shall
Rootech Inc. be liable for consequential damages sustained in connection with said
product and Rootech Inc. neither assumes nor authorizes any representative or other
person any obligation or liability other than such as is expressly set forth herein.
The information contained in this document is believed to be accurate at the time of
publication, however, Rootech Inc. assumes no responsibility for any errors which may
appear here and reserves the right to make changes without notice.
Page 3
Standard Compliance
CE Approved
EN61326 and IEC61010
35DX
MEASURING EQUIPMENT
E258934
KSA 9001:2001/ ISO 9001:2001
QMS-1347
Page 4
Warranty Information
For products and software that are sold or licensed by Rootech Inc. during the period
from the date of receipt by you until the present, Rootech warrants only to the original
purchaser.
The purchased products other than software shall be substantially free from material
defects in material and workmanship by Rootech for two years from the date receipt by
you.
The software itself is provided “as is” without any warranty of any kind.
Limitation of Warranty
These limited Warranties shall not apply to any product that has been subject to
alteration, accident, misuse, abuse, neglect or failure to exactly follow Power
Measurement's instructions for operation and maintenance.
In order for the original purchaser to make a claim under the warranties described
above, the original purchaser must promptly contact Rootech headquarter. After
receiving such notice, Rootech may either in Rootech’s sole discretion, examine the
product at the original purchaser’s site or issue shipping instructions to the original
purchaser to return the relevant product to Rootech for examination at the original
purchaser’s expense, transportation charges prepaid.
If after examining the product Rootech reasonably confirms that such examined
product does not meet the warranties, then the original purchaser’s sole remedy and
Rootech’s sole obligation or liability shall be, at Rootech’s option, to repair, replace or
refund the price paid for that product.
Regardless of whether any remedy set forth herein fails of its essential purpose, except to
the extent the following limitation is prohibited by applicable law, Rootech shall not, in
any event or under any legal claim or theory, be liable to the original purchaser or any
other person or entity for special, indirect, incidental, punitive, liquidated, special or
consequential damages whatsoever with respect to any purchased product, including,
without limitation, business interruption, loss of use, profit or revenue, even if Rootech has
Page 5
been advised of the possibility of such damages. To the extent that a limitation or
exclusion of consequential damages are prohibited by applicable law, then Rootech’s
liability shall be limited to twice the amount of the relevant purchased product.
Rootech shall not be liable for any claim- other than a claim solely for the breach of one
of the above warranties that is made in accordance with the above described
procedures- made by the original purchaser, its employees, agents, or contractors for
any loss, damage, or expense incurred due to, caused by, or related to any purchased
product
Any technical assistance provided by Rootech's personnel or representatives in system
design shall be deemed to be a proposal and not a recommendation. The responsibility
for determining the feasibility of such proposals rests with the original purchaser and
should be tested by the original purchaser.
It is the original purchaser’s responsibility to determine the suitability of any product and
associated documentation for its purposes. The original purchaser acknowledges that
100% "up" time is not realizable because of possible hardware or software defects. The
original purchaser recognizes that such defects and failures may cause inaccuracies or
malfunctions.
Only the terms expressed in these limited Warranties shall apply and no distributor,
corporation or other entity, individual or employee of Rootech or any other entity is
authorized to amend, modify or extend the Warranties in any way.
Page 6
Revision History
The following versions of the Accura 3500 Communication Guide have been released.
Revision #
Revision date
Description
Firmware version
Revision 1.0
2006. 06. 30
Initial release
Revision 1.1
2007. 04. 24
Added Short-formed data block
1.18
Added a register of extension
module configuration.
1.22
Revision 1.2
2007. 07. 13
Figure and comm examples
modified
Revision 1.3
2024. 01. 12
Added Energy of Quadrant 4
Corrected typos in the main text
and bookmark
Page 7
Contents
Chapter 1 Serial Communication .................................................................................................................... 9
Overview ........................................................................................................................................................................................... 9
Master/Slave Multi-drop ................................................................................................................................................. 9
Modbus RTU Protocol ..................................................................................................................................................... 9
Communication ........................................................................................................................................................................... 10
RS485 Communication Timing .................................................................................................................................. 11
Packet Recognition Time .............................................................................................................................................. 11
Holding Time before transmitting Request Packet ......................................................................................... 11
Chapter 2 Communication Setup ................................................................................................................. 12
Communication Wiring ............................................................................................................................................................ 12
Setup ................................................................................................................................................................................................. 13
Setup Mode ........................................................................................................................................................................ 13
Chapter 3 Modbus RTU Protocol ............................................................................................................... 16
Packet Field of Modbus RTU ................................................................................................................................................ 16
Slave Address Field ......................................................................................................................................................... 16
Function Field..................................................................................................................................................................... 16
Data Field ............................................................................................................................................................................. 16
Error Check Field .............................................................................................................................................................. 17
Exception Responses ...................................................................................................................................................... 17
Broadcast Packets............................................................................................................................................................. 18
Packet Communications .......................................................................................................................................................... 19
Function 03: Read Holding Registers ..................................................................................................................... 19
Function 06: Write Single Register to Meter ...................................................................................................... 20
Function 16: Write Multiple Registers to Meter ............................................................................................... 20
Reserved Registers........................................................................................................................................................... 21
Invalid Registers ................................................................................................................................................................ 21
Chapter 4 Modbus RTU Map ...................................................................................................................... 22
Overview ......................................................................................................................................................................................... 22
System information section ................................................................................................................................................... 24
Configuration section ............................................................................................................................................................... 25
Measurement section ............................................................................................................................................................... 27
THD, k Factor section ................................................................................................................................................................ 29
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Extra energy section .................................................................................................................................................................. 29
Demand, Maximum, Minimum section ........................................................................................................................... 30
Harmonic section ........................................................................................................................................................................ 33
Vector diagram section† .......................................................................................................................................................... 34
Waveform section....................................................................................................................................................................... 35
Demand trend section ............................................................................................................................................................. 37
Reset section ................................................................................................................................................................................. 37
DIO module section† ................................................................................................................................................................. 38
DI module section† .................................................................................................................................................................... 39
DO module section† .................................................................................................................................................................. 40
AI module section† ..................................................................................................................................................................... 41
AO module section† .................................................................................................................................................................. 41
Short-formed data block Section ....................................................................................................................................... 42
Data formats ................................................................................................................................................................................. 46
Parameter Calculation .............................................................................................................................................................. 48
Fixed scale ........................................................................................................................................................................... 48
Variable scale ...................................................................................................................................................................... 48
APPENDIX A CRC-16 Calculation ................................................................................................................ 50
CRC-16 Generation .................................................................................................................................................................... 50
CRC Generation Procedure.......................................................................................................................................... 50
CRC table ............................................................................................................................................................................. 51
APPENDIX B Modbus RTU C Code Example ............................................................................................ 52
Data Receiving and CRC Generation& Checking ....................................................................................................... 52
Request Packet Generation ......................................................................................................................................... 52
Response Packet Checking .......................................................................................................................................... 52
CRC functions ..................................................................................................................................................................... 53
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Chapter 1 Serial Communication
Overview
Accura 3500 has a RS485 port on the rear side of meter for Master/Slave communication
in the field, and supports Modbus RTU protocol.
Modbus RTU protocol is a industry standard to efficiently transfer information of
measuring and control between master[PC or PLC] and slave[local devices, Accura
3300/3500/3550/7500].
Master/Slave Multi-drop
◼ All communications on network conforms to a Master/Slave scheme.
◼ Master will initiate and control all information on the communications loop.
◼ Under no circumstances A Slave device initiate a communications sequence.
◼ All communications occurs in the form of Packet, a packet being simply a serial
string of 8-bit bytes.
◼ All Packets transmitted by the Master are requests. All Packets transmitted by a
Slave are responses.
◼ At most one Slave can respond to a single request from a Master.
Modbus RTU Protocol
◼ Industry standard protocol for general users.
◼ Modbus RTU is recommended.
◼ Designed according to Modicon Modbus RTU specifications.
◼ Provides selective data communications by address map.
◼ Most of automation programs provides the driver for Modbus RTU.
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Communication
The default serial communication setup of Accura 3500 is 8 data bits, even parity, 1 stop
bit.
The RS485 communications allows multiple devices to be connected on the same bus.
Up to 32 devices can be connected on a single RS485 bus, which consists of a shielded
twisted pair cable more than AWG 22[0.33mm2]. The overall length of the RS485 cable
connecting all devices cannot exceed 1.2km[ or 4000ft].
RS485 Multi-drop Connection Diagram
Warning
The above T bridge – branched off in the middle of communication line- cannot
guarantee the communication reliability by voltage-drop on the line.
To connect an RS485 communication bus to a computer or other RS232 equipped
device, a RS232 to RS485 converter is required. The communication between computer
and Accura 3500 is to connect the Pins, Ta, Tb of the Accura 3500 to the RS485 two lines,
Data+, Data-, and between computer and converter is connected one to one. The
timing flow Between request packet and response packet is followed.
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RS485 Communication Timing
The figure below shows the timing delay between the Request Packet and the Response
Packet.
Packet Recognition Time
The data packet between master[PC] and slave[devices] has the following format. One
packet consists of consecutive data frames, and frames is recognized as the same
packet for the condition where the distance between frames is within 5ms. At least 10ms
is required to distinguish each packet.
Holding Time before transmitting Request Packet
After receiving the data from meter, at least 10 msec[particularly 13 msec in 57600bps]
is required to send a next request packet.
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Chapter 2 Communication Setup
Communication Wiring
Communication Wiring
Pin name
Ta[TRX+], Tb[TRX-]
Connector
Screw terminal[Pluggable]
Wire
1.25 to 3.5 mm2[24 to 14 AWG], Shielded twisted pair
Maximum Cable Length
1219 m[4000 ft]
Maximum Devices/Bus
32
Note
UL2919 cable is recommended for communication.
Comm Indicator
Comm LED shows green when the meter is transmitting data.
Page 13
Setup
Each of these functions can be accomplished by pressing the Menu, Up, Down, and
Enter buttons on the front panel.
Display mode
View parameter measurements
Setup mode
Configure parameters[wiring, communication, and etc.]
Reset mode
Reset energy, demand, maximum, and minimum.
System mode
View meter system information[meter option, firmware
version].
Module mode
View information of module added on rear side of the meter
Note
SETUP LED shows red when selecting Setup, Reset, System, and Module modes.
Setup Mode
In Setup mode, you can setup menus, such as PT/CT ratio, wiring, and communication
after mounting the meter.
Button Functions in Setup Mode
Button
Function
Enter setup mode by pressing the Up and Down buttons at
the same time.
Move to the previous menu screen.
Move to the next menu screen.
Enter setup edit mode to configure the displayed menu
value.
→ the displayed value blinks†
Exit setup mode and move to display mode.
†It shows that the value of menu item is possible to be changed.
Page 14
Button Functions in Setup Edit Mode
Button
Function
†
Increase the value of menu item.
‡
decrease the value of menu item.
Save the edited value of menu item, and the meter
automatically returns to Setup Mode.
†When pressing at least two seconds, cursor is moved to higher position.
‡When pressing at least two seconds, cursor is moved to lower position.
Note
In setup edit mode, pressing button exits system edit mode and moves to
display mode, without saving the changed value of menu item.
Menu Items
Order
Menu Item
Range
Default
1
PT ratio
0.1 ~ 999.9
PT ratio =
PT primary/ secondary
1.0
2
CT ratio
1 ~ 5,000
CT ratio =
CT primary/ secondary
10
3
Wiring mode
Single phase 2 wires
Single phase 3 wires
Three phase 3 wires,
Open Delta
Three phase 4 wires
√
4
Comm address
1 ~ 247
5
Protocol
Modbus RTU
√
Rootech†
6
Speed[bps]
1200bps
2400bps
4800bps
9600bps
√
19200bps
38400bps
57600bps‡
Page 15
7
Parity bits
Even parity
√
Odd parity
None parity
8
Stop bits
1 ~ 2
1
†Available only for manufacturer.
‡In 57600bps, after receiving the data from meter, at least 13msecs is required to send a next
request packet. And when writing multiple registers to meter, the size of data word is limited to 24
words.
Page 16
Chapter 3 Modbus RTU Protocol
Accura 3500 supports Modbus RTU.
Packet Field of Modbus RTU
Every modbus packet consists of four fields.
◼ Slave Address Field
◼ Function Field
◼ Data Field
◼ Error Check Field
Slave Address Field
The slave address field of a Modbus packet is one byte in length and uniquely identifies
the slave device involved in the transaction. Valid addresses range between 1 and 247.
A slave device, when receiving a request packet with the slave address field matching
its own address, will perform the command specified in the packet. A response packet
generated by the slave will have the same value in the slave address field.
Function Field
The function field of a Modbus request packet is one byte in length and tells the
addressed slave which function to perform. Similarly, the function field of a response
packet tells the master what function the addressed slave has just performed.
Function[Decimal]
Meaning
Description
03
Read Multiple register
Obtains the current value from registers
06
Write Single register
Updates one single register to setup value
16
Write Multiple registers
Updates more than a register to setup
value
Data Field
The data field of a Modbus request is of variable length, and depends upon the
function. This field contains information required by the slave device to perform the
command specified in a request or data being passed back by the slave device in a
Page 17
response packet. Data in this field is contained in 16-bit or 32-bit registers. Registers are
transmitted in the order of high-order byte first, low-order byte second. This ordering of
bytes is called “Big Endian” format.
Big Endian Example]]
High order byte = 3A hex
Low order byte = 12 hex
This register is transmitted in the order of 12AB
Error Check Field
The checksum field allows the receiving device to determine if a packet has been
corrupted with transmission errors. In Modbus RTU mode, 1 16-bit cyclic redundancy
check[CRC-16] is used. The sending device calculates a 16-bit value using the CRC-16
algorithm. The calculated value is inserted in the error check field.
The receiving device performs the same calculation on the entire packet it receives,
excepting the error check field. If the calculated checksum is not equal to the
checksum stored in the incoming packet, transmission errors occurred and corrupted
the packet. A bad packet will be ignored by the receiving device. Refer to the
“Appendix A CRC-16 Calculation” for details of CRC-16 algorithm.
Packet Format
Slave address
Function
Data
Error check
1 byte
1 byte
N 2 bytes
2 bytes
Exception Responses
If a Modbus master device sends an invalid command to Accura 3500 or attempts to
read an invalid holding register, an exception response will be generated. The high
order bit of the function code in an exception response is set to 1.
The data field of an exception response contains the exception error code.
Exception Response
Slave address
Function
Exception code
Error check
1 byte
1 byte
1 byte
2 bytes
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Exception code
Name
Description
01
Illegal Function
Unavailable command in Request Packet
02
Illegal Address
Unavailable holding address in Request Packet
03
Illegal Value
Unavailable data transfer in Holding Address
Broadcast Packets
The purpose of a broadcast request packet is to allow all the slave devices to receive
the same command from the master. A broadcast request packet is the same as a
normal request packet, except the slave address field is set to zero.
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Packet Communications
This section illustrates the Modbus functions supported by the meter.
Function 03: Read Holding Registers
To read meter parameter values, a Master must send the slave device a Read Holding
Registers request packet. The Read Holding Registers request packet specifies a start
register and a number of registers to read.
Starting Address is a relative address according to register 40000[40001=0, 40002=1,
40101= 100, and etc.]. refer to Appendix B for CRC.
Request
Slave
Function
Starting address
Word count
Error check
03
1 byte
1 byte
2 bytes
2 bytes
2 bytes
Example]
A meter is configured as a Modbus slave device with slave address 1. A phase voltage
has the address 40101, and B phase voltage 40102. register 40101 is numbered as 100
when transmitted.
slave address = 1, starting address = 64, word count = 2, crc high = 85, crc low = D4
Slave
Function
Starting Address
Word Count
Error Check
01
03
00
64
00
02
85
D4
Response
Slave
Function
Byte Count
Data Word 1
---
Data Word N
Error Check
03
---
1 byte
1 byte
1 byte
2 bytes
---
2 bytes
2 bytes
Data word represents the data required in the request packet. Byte count is two times
word count.
Example]
Slave device with address 1 has A phase voltage= 1A1B, B phase voltage= 223B.
slave address = 1, byte count = 4, crc high = D4, crc low = 5F
Page 20
Slave
Function
Byte Count
Data Word 1
Data Word N
Error Check
01
03
04
1A
1B
22
3B
D4
5F
Function 06: Write Single Register to Meter
To write data to a single register, a master must send the slave device a write single
register to meter request packet.
Request
Slave
Function
Starting Address
Data Word
Error Check
06
1 byte
1 byte
2 bytes
2 bytes
2 bytes
Example]
Writes data[78hex] to PT ratio [40053] of the meter with slave address 1.
slave address = 1, starting address = 34, data word = 78, crc high =C8, crc low = 26
Slave
Function
Starting Address
Data Word
Error Check
01
06
00
34
00
78
C8
26
Response
The response packet to Function 06 command is to send the request packet reversely.
Example]
The response to the request packet is as follows.
Slave
Function
Starting Address
Data Word
Error Check
01
06
00
34
00
78
C8
26
Function 16: Write Multiple Registers to Meter
To write data to the multiple registers, a Master must send the slave device a Write
Multiple Registers to Meter request packet.
Request
Slave
Function
Starting Address
Word Count
Byte Count
10
1 byte
1 byte
2 bytes
2 bytes
1 byte
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