GFC 300 Supplementary instructions
Signal converter for gas ultrasonic flowmeters
Description of Modbus interface
Electronic Revision: ER 1.0.xx (SW.REV. 1.0.x)
Modbus version 1.0.x
© KROHNE 03/2012 - 7312402100 - AD Modbus GFC 300 R01 en
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CONTENTS |
GFC 300 |
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1 |
Important information |
3 |
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2 |
Technical data |
4 |
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2.1 |
General technical data ..................................................................................................... |
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2.2 |
Technical data of the Modbus interface (acc. to EIA standards) ..................................... |
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Connection in bus systems |
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4 |
Local configuration |
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Electrical connection |
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6 |
Modbus protocol |
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6.1 |
General information concerning the protocol ................................................................. |
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6.2 |
RTU frame format............................................................................................................. |
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6.3 |
Addressing...................................................................................................................... |
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6.4 |
Overview of supported functions.................................................................................... |
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6.5 |
Device identification on the Modbus interface............................................................... |
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6.6 |
Coil registers .................................................................................................................. |
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6.6.1 Converter controls ................................................................................................................ |
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6.6.2 Counter controls ................................................................................................................... |
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6.6.3 Start calibration functions .................................................................................................... |
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6.7 |
Input registers ................................................................................................................ |
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6.8 |
Holding registers............................................................................................................ |
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6.8.1 Counter parameters ............................................................................................................. |
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6.8.2 Process input parameters .................................................................................................... |
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6.8.3 Modbus parameters.............................................................................................................. |
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6.9 |
Diagnostics ..................................................................................................................... |
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7 |
Notes |
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18 |
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2 |
www.krohne.com |
03/2012 - 7312402100 - AD Modbus GFC 300 R01 en |
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IMPORTANT INFORMATION 1 |
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GFC 300 |
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The flow converter with the RS485 interface card fitted, is able to communicate with an external device (PC or other suitable computer system) using the Modbus protocol. This option allows data exchange between PC or computer and single or multiple devices.
The bus configuration consists of one external device as a master and one or more converters as slaves. For bus operation the device address (menu C5.8.1), baudrate (menu C5.8.2) and settings (menu C5.8.3, C5.8.4, C5.8.5 & C5.8.6) must be set in the converter.
All devices connected to the bus, must have different unique addresses but the same baud rate and settings.
03/2012 - 7312402100 - AD Modbus GFC 300 R01 en |
www.krohne.com |
3 |
2 TECHNICAL DATA |
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GFC 300 |
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2.1 General technical data |
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Interface |
RS485, galvanically isolated |
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Baud rate |
1200, 2400, 3600, 4800, 9600, 19200, 38400, 57600 or 115200 |
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Protocol |
Modbus RTU (available as a separate document on request) |
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Maximum participants on bus |
32 per line, master included (may be extended by repeaters) |
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Coding |
NRZ bit coding |
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Address range |
Modbus: 1...247 |
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Transmission procedure |
Half duplex, asynchronous |
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Bus access |
Master / slave |
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Cable |
Shielded twisted pair for RS 485 applications |
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Distances |
Maximum 1.2 km / 3937 ft without repeater (dependant on |
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baud rate and cable specifications) |
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2.2 Technical data of the Modbus interface (acc. to EIA standards)
Kind of signal transmission |
Differential, 2-wire topology |
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Maximum number of |
32 |
transmitter/receivers |
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Voltage range on converter input |
-7...+12 V |
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Maximum voltage on converter output |
5 V |
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Minimum voltage on driver output, max. |
Udiff > 1.5 V |
load |
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Maximum input current (off state) |
-20...+20 μA |
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Receiver input voltage |
-7...+12 V |
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Sensitivity of the receiver |
-200...+200 mV |
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Receiver input resistance |
> 12 kΩ |
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Short circuit current |
< 250 mA |
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Termination / polarization resistors |
120 Ω / 560 Ω |
(if activated by the jumpers X5/X6) |
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4 |
www.krohne.com |
03/2012 - 7312402100 - AD Modbus GFC 300 R01 en |
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CONNECTION IN BUS SYSTEMS 3 |
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GFC 300 |
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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, normally the "end".
The Modbus converter has two conditions. Default is without termination and polarization. To get the active termination and polarization the settings of jumper X5 and X6 on Modbus board must be changed. This can only be done by qualified personnel. For detailed information, refer to
Electrical connection on page 7.
03/2012 - 7312402100 - AD Modbus GFC 300 R01 en |
www.krohne.com |
5 |
4 LOCAL CONFIGURATION |
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GFC 300 |
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Converter Fct. |
Display |
Description and settings |
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No. |
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C5.8.1 |
Slave Address |
Selects the Modbus address of the device. |
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Range: 1..247 (default = 1) |
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C5.8.2 |
Baud Rate |
Selects the baud rate of the device. |
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Options: |
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1200 / 2400 / 3600 / 4800 / 9600 / 19200 (default) / 38400 / 57600 / 115200 |
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C5.8.3 |
Parity |
Selects the parity. |
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Options: |
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Even (default) / Odd / No |
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C5.8.4 |
Data Format |
Selects the data format. |
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Options: |
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Big Endian (default) / Little Endian |
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C5.8.5 |
Transmission Delay |
Selects the delay between receiving the last byte of a request and |
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sending the first byte of the response. |
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Range: 0..40ms (default = 0ms) |
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C5.8.6 |
Stop Bits |
Selects the number of stop bits. |
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Options: |
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1 (default) / 2 |
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C5.8.7 |
Information |
Displays information about the device. |
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6 |
www.krohne.com |
03/2012 - 7312402100 - AD Modbus GFC 300 R01 en |
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