KROHNE MFC-400 User Manual

MFC 400 Supplementary instructions

Signal converter for mass flowmeters

Description of Modbus interface

Electronic Revision: ER 1.0.3_

Modbus version 2.1.0_

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	CONTENTS	MFC 400

1 General information			4
1.1	Scope of the document.....................................................................................................		4
1.2	Scope of delivery...............................................................................................................		4
1.3	Modbus protocol interface ...............................................................................................		4
2 Technical data			5
2.1	General technical data .....................................................................................................		5
2.2	Technical data of the Modbus interface (acc. to EIA/TIA standards) ..............................		5
3 Electrical connections			6
3.1	Modbus connection...........................................................................................................		6
3.2	Connection to Modbus bus ...............................................................................................		6
4 Establish RS485 connection			7
5 Modbus protocol			8
5.1	RTU frame format.............................................................................................................		8
5.2	Data representation .........................................................................................................		9
5.2.1		8-bit values..............................................................................................................................	9
5.2.2		16-bit values............................................................................................................................	9
5.2.3		32-bit values............................................................................................................................	9
5.2.4		64-bit values..........................................................................................................................	10
5.3	Modbus Register Addresses ..........................................................................................		11
5.4	Supported Function Codes .............................................................................................		11
5.5	Error messages..............................................................................................................		11
5.6	Device identification .......................................................................................................		12
5.7	Diagnostics .....................................................................................................................		13
5.8	Parameters.....................................................................................................................		13
5.8.1 Device Control.......................................................................................................................			14
5.8.2 Device Status.........................................................................................................................			15
5.8.3 Measurement values ............................................................................................................			16
5.8.4 Measurement Status compliant with NAMUR NE 107 .........................................................			16
5.8.5		Auxiliary Values.....................................................................................................................	17
5.8.6		Totaliser ................................................................................................................................	17
5.8.7 Zero Calibration ....................................................................................................................			20
5.8.8		Density Calibration Coefficients ...........................................................................................	21
5.8.9 Density Configuration ...........................................................................................................			21
5.8.10 Filters ..................................................................................................................................			22
5.8.11 System Control....................................................................................................................			22
5.8.12 Calibration Coefficients ......................................................................................................			23
5.8.13 Modbus RS485 Communication Settings ...........................................................................			24
5.8.14 NAMUR NE 107 Variable Event Group(s)............................................................................			25
5.8.15 Concentration 1...................................................................................................................			26
5.8.16 Concentration 2...................................................................................................................			27

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					CONTENTS
	MFC 400
6			Application sequences		28
			6.1	Saving and restoring the configuration setting .............................................................	28
			6.2	Zero calibration ..............................................................................................................	29
7			Troubleshooting		30
			7.1	No response to Modbus requests ..................................................................................	30
			7.2	Communication errors ...................................................................................................	30
			7.3	Responding with exception "Illegal Function"...............................................................	30
			7.4	Responding with exception "Illegal Data Address" .......................................................	31
			7.5	Responding with exception "Illegal Data Value"............................................................	31
8			Appendix		32
			8.1	NAMUR NE 107 Event Group(s)......................................................................................	32
			8.2	NAMUR NE 107 status signals .......................................................................................	33
			8.3	Supported Modbus function codes.................................................................................	33
			8.4	Number format...............................................................................................................	36
			8.5	Glossary ..........................................................................................................................	36
9			Notes		37

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3

1 GENERAL INFORMATION
	MFC 400

1.1 Scope of the document

These instructions are supplementary to the signal converter handbook. For all other data, use the relevant chapters of the handbook. If you do not have this document, please contact the nearest office or download them from the manufacturer's internet site.

1.2 Scope of delivery

The information in this supplementary manual only contains the data applicable to MODBUS communication.

The technical data in the handbook shall be valid in its current version, provided that it is not rendered invalid or replaced by this supplement.

1.3 Modbus protocol interface

The Modbus interface to the signal converter is implemented in the Modbus RTU communications protocol and is done in accordance with the specification and requirements of the "Modbus Protocol Specification V1.1b".

The physical electrical parameters of the Modbus specification are defined by the EIA/TIA-485 (RS485) standard and the "Modbus over Serial Line - Specification and Implementation Guide V1.02".

Both specifications can be obtained on the official website of the Modbus organisation:

http://www.modbus.org

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				TECHNICAL DATA 2
	MFC 400
	2.1 General technical data
		Interface	RS485, galvanically isolated
		Baud rate	1200, 2400, 3600, 4800, 9600, 19200, 38400, 57600 or
			115200 bps
		Protocol	Modbus RTU (available as a separate document on request)
		Maximum participants on bus	32 per line, master included (may be extended by repeaters)
		Coding	NRZ bit coding
		Address range	1...247
		Transmission procedure	Half duplex, asynchronous
		Bus access	Master / slave
		Cable	Screened twisted pair
		Distances	Maximum 1.2 km / 3937 ft without repeater (dependant on
			baud rate and cable specifications)

2.2 Technical data of the Modbus interface (acc. to EIA/TIA standards)

Kind of signal transmission	Differential, 2-wire topology
Maximum number of	32
transmitter/receivers
Voltage range on converter input	-7...+12 V
Maximum voltage on converter output	5 V
Minimum voltage on driver output, max.	Udiff > 1.5 V
load
Maximum input current (off state)	-20...+20 μA
Receiver input voltage	-7...+12 V
Sensitivity of the receiver	-200...+200 mV
Receiver input resistance	> 12 kΩ
Short circuit current	< 250 mA
Termination / polarization resistors	120 Ω / 560 Ω
(if activated by factory)

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3 ELECTRICAL CONNECTIONS
	MFC 400

3.1 Modbus connection

The signal converter is hooked up onto the bus using terminals C and D:

Terminals	Description
D-	Signal A (D 0)
D	Signal B (D 1)
C-	Common 0 V
C	Not connected

Terminals A and B of the signal converter are dependant on the options selected at order. Refer to the standard handbook of the signal converter for connection details.

3.2 Connection to Modbus bus

The signal converter is designed to be connected as a slave device onto the 2-wire bus implementation of the Modbus serial physical layer definition.

In addition to the D0 and D1 signal lines the bus MUST include a "Common" signal line to act as a ground reference point for the data signals.

For proper operation of Modbus in half duplex mode in single or multi-drop communication, it is recommended that a termination resistor is applied to both ends of the data line. The simplest form of termination is line-to-line resistor across the differential input.

In RTU mode the Modus protocol requires quiet periods on the communications bus for synchronisation. It is therefore important that the Modbus is not allowed to "float", i.e. unreferenced to 0 V, as this could lead to spurious signals due to noise pick-up. It is therefore necessary to employ biasing resistors at one point on the bus network.

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		ESTABLISH RS485 CONNECTION 4
	MFC 400

To establish a RS485 connection with the signal converter, prepare the master device with the appropriate default settings or use custom parameters specified via display of the signal converter.

The Modbus RS485 settings can be found in the menu C6.8 of the display. The following parameters can be configured:

Parameter	Legal values	Default values	Display Fct. No.
Slave address	1...247	1	C6.8.1
"Baude rate"	1200, 2400, 3600, 4800, 9600,	19200 bps	C6.8.2
	19200, 38400, 57600, 115200
	bps
Parity	Even, Odd, No	Even Parity	C6.8.3
Data format	Big Endian, Little Endian	Big Endian	C6.8.4
Transmission	0...0.04 [s]	0 s	C6.8.5
delay
Stop Bits	1 Stop Bit, 2 Stop Bits	1 Stop Bit	C6.8.6

These settings can be changed via Modbus too. For further information refer to Modbus RS485 Communication Settings on page 24.

All devices connected to the bus, must have the same baud rate.

INFORMATION!

It is of great importance to ensure at the time of the procedure of devices addressing, that there are not two devices with the same address. In such a case, an abnormal behaviour of the whole serial bus can occur. It is then impossible for the master to communicate with all present slaves on the bus.

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5 MODBUS PROTOCOL

5.1 RTU frame format

MFC 400

Using RTU (Remote Terminal Unit) format, data is transmitted as 8 bit binary characters. There are no special characters to determine the start and end of a message frame. Synchronization is achieved by a minimum silent period of at least 3.5 character times before the start of each frame transmission and a maximum silent period of 1.5 character times between characters in the same frame.

The format of the query and response frames vary slightly depending upon the function code. The basic form is outlined below.

Command function	Frame format	Description
Silent period	3.5 x T	All transmissions must be preceded by a minimum silent
		period of 3.5 x T, where T is the transmission time of a
		single character. This can be calculated from the baud
		rate, e.g. T = 572 µs at 19.2 kbps.
Slave address	8 bits	This is a single byte slave address which is transmitted
		first and must be in the range of 1...247. Address 0 is
		reserved for a broadcast address which all slaves should
		recognise, and therefore requires no response.
Function code	8 bits	This is an eight bit code in the range of 1...255 although
		only 126 functions exist as the codes 129...255 represent
		an error condition. An error condition occurs when the
		addressed slave does not accept the command, in which
		case it responds with the function code + 128, i.e. with its
		msb set to 1.
Register start address or	8 bit byte count	Register start address: for a query command that requires
byte count when required	16 bit address	data to be returned, this field will contain the 16 bit start
		address of the register (or data) to be returned.
		Note that the signal converter uses protocol addresses.
		Therefore the register address listed is the actual number
		required in the Modbus command.
		E.g: to access input register 30006, the register start
		address is 30006 = 0x7536.
		Byte count: In general this is only present in frames that
		are transferring data, and has a value equal to the number
		of bytes contained in the data field. The data field is limited
		to a maximum of 250 bytes.
Number of points or data	n × 8 bits	Number of points: for a query command that requires data
bytes when required		to be returned, this field will contain the number of
		registers to be returned regardless of their bit size.
		Data bytes: contains the data requested. The signal
		converter can use Big Endian format (MSB first) or Little
		Endian format (LSB first).
CRC	16 bits	This field contains a 16 bit CRC which is calculated on all
		the data bits of the message bytes.

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		MODBUS PROTOCOL 5
	MFC 400

5.2 Data representation

There are two data types used to transmit information on a Modbus data bus, the "Bit" and the "Register". The "Bit" represents a single binary state, whether as an output or an input condition. The "Register" is a 16-bit integer transmitted as two 8-bit characters. Using multiple "Registers" the Modbus interface can transmit higher accuracy values such as "Floating Point" and "Double Precision Floating Point" numbers.

"Bit" variables are packed into a byte containing 8 bit, so each character, sent or received, can contain up to 8 "Bit" variables. The master and slave devices use only as many 8 bit data characters as are required to transmit the information. Any unused bits in the data characters are ignored. The bit that is requested by the start address is transmitted in the LSB at bit 0. The next "Bit" value is transmitted in the next bit (bit 1). This continues until the last bit location (bit 7) of the LSB is reached. The next "Bit" value is then transmitted in the next data byte (LSB+1/MSB) at bit 0. This continues until all of the requested values have been transmitted. Any unused bits in the MSB are filled out with "0"s.

For simple single register variables the MSB of the register is transmitted first, with the LSB following immediately after. However, for variables that require multiple registers, i.e. the "Floating Point" and "Double Precision Floating Point" variables, the transmission order can be selected in the RS485 settings. By default, those values will be transmitted in Big Endian.

5.2.1	8-bit values
		Register	Hi		Lo
		N		0x00	Byte
5.2.2	16-bit values
		Register Hi			Lo

NMSB LSB

5.2.332-bit values

Little Endian

Register	Hi	Lo
N	LSB + 1	LSB
N + 1	MSB	LSB + 2
Big Endian
Register	Hi	Lo
N	MSB	LSB + 2
N + 1	LSB + 1	LSB

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5 MODBUS PROTOCOL

MFC 400

Float (single precision, IEEE 754)

MSB	LSB + 1	LSB + 1	LSB
SEEEEEEE	EMMMMMMM	MMMMMMMM	MMMMMMMM

With S = sign, E = exponent, M = mantissa/fraction

5.2.4 64-bit values

Little Endian
Register	Hi	Lo
N	LSB + 1	LSB
N + 1	LSB + 3	LSB + 2
N + 2	LSB + 5	LSB + 4
N + 3	MSB	LSB + 6
Big Endian
Register	Hi	Lo
N	MSB	LSB + 6
N + 1	LSB + 5	LSB + 4
N + 2	LSB + 3	LSB + 2
N + 3	LSB + 1	LSB

Double (double precision, IEEE 754)

MSB	LSB + 6	LSB + 5	LSB + 4
SEEEEEEE	EEEEMMMM	MMMMMMMM	MMMMMMMM
LSB + 3	LSB + 2	LSB + 1	LSB
MMMMMMMM	MMMMMMMM	MMMMMMMM	MMMMMMMM

With S = sign, E = exponent, M = mantissa/fraction

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		MODBUS PROTOCOL 5
	MFC 400

5.3 Modbus Register Addresses

The signal converter supports four types of data references, which are associated to a range of Modbus registers.

Address range	Primary tables	Access rights
0...9999	Coils	read + write
10000...19999	Discrete Inputs	read
20000...39999	Input Registers	read
40000...65535	Holding Registers	read + write

INFORMATION!

•Sometimes register numbers are asked for. The register numbers can be calculated by adding a 1 to the register address.

•Some systems cannot use addresses above 9999. For these systems there is the possibility to use the listed addresses but

-for Input Registers omit the leading 3 of 3xxxx;

-for Holding Registers omit the leading 4 of 4xxxx.

5.4Supported Function Codes

Function code		Name
dec	hex
01	01	Read Single Coil
02	02	Read Descrete Inputs
03	03	Read Holding Register
04	04	Read Input Register
05	05	Write Single Coil
08	08	Diagnostics
16	10	Write Multiple Register
43	2B	Encapsulated Interface Transport

For detailed information about the telegrams structure of all function codes refer to Supported Modbus function codes on page 33.

5.5 Error messages

When the signal converter detects an error in the requests, received in a properly formatted telegram, it will respond with an error message. The error message response telegram is formatted as follows:

Address	Function Code	Error Code	CRC Lo	CRC Hi

The msb (most significant bit) of the requested function code is set (add 0d128 / 0x80) in the reponse telegram to indicate an error has been detected. For example, if an error were detected in a function 1 request, then the returned function code would be 0x81 (0d129).

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5 MODBUS PROTOCOL

MFC 400

The single data character in the response telegram will indicate the type of error detected.

These are as follows:

Error	Name	Meaning
Code
01	ILLEGAL FUNCTION	The requested function code is not supported or not valid due to the
		current settings of the device.
02	ILLEGAL DATA	The register requested is not valid or the quantity of requested
	ADDRESS	registers hits invalid registers.
03	ILLEGAL DATA VALUE	The requested data is invalid for the register being written.
04	SLAVE DEVICE	An unrecoverable error occurred while the slave was attempting to
	FAILURE	perform the requested action.
06	SLAVE DEVICE BUSY	The slave is unable to process the requested command because a
		long-duration command is in progress. The master should
		retransmit the message later.

Errors due to communications faults (CRC errors, Parity errors etc.) are logged but no response is returned because the data in the received telegram is deemed unreliable. The master system can read the error logs by using the diagnostics command (for details on Function Code 0x08 refer to Diagnostics on page 13).

5.6 Device identification

Retrieve all of the identification information from the signal converter.

Modbus Function Code "Encapsulated Interface Transport" (0x2B).

Modbus Encapsulated Interface (MEI) type (0x0E).

Category	Object	Object name	Type	Content
	Id
Basic	0x00	VendorName	16 byte ASCII String	KROHNE
	0x01	ProductCode	10 byte ASCII String	CG number; order code for the
				signal converter assembly
	0x02	MajorMinorRevision	7 byte ASCII String	Electronic Revision number
Regular	0x03	Vendor URL	32 byte ASCII String	www.krohne.com
	0x04	ProductName	16 byte ASCII String	MFC400
	0x05	ModelName	16 byte ASCII String	Modbus
	0x06	UserApplicationName	16 byte ASCII String	User tag, displayed on the
				header of the local screen

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