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Modbus

Here you can find out about Modbus, answers to the frequently asked questions and some trouble shooting tips

What is Modbus?

Modbus is a communication protocol developed by Modicon systems. In simple terms, it is a method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves. In a standard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The Master can also write information to the Slaves. The official Modbus specification can be found at www.modbus.org.

What is it used for?

Modbus is an open protocol, meaning that it's free for manufacturers to build into their equipment without having to pay royalties. It has become a very common protocol used widely by many manufacturers throughout many industries. Modbus is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system.

How does it work?

Modbus is transmitted over serial lines between devices. The simplest setup would be a single serial cable connecting the serial ports on two devices, a Master and a Slave.



Most frequently asked questions...

  1. Do I need terminator resistors on the cables?
    Yes. The terminator resistors indicate the beginning and the end of the LAN. So it is imperative terminator resistors (120 ohm) have to be fitted. The last Meter on the LAN and the Horizon or management system (see brochure HC1).
  2. Can I use the same modbus address on more than one meter?
    No. Each meter must have its own unique address (1-255)
  3. How long can each LAN be?
    The maximum length of cable on each LAN should not exceed 1Km
  4. How many meters can I fit on a LAN?
    The maximum number of meters per LAN is 255
  5. How is it connected?
    Please see the appropriate brochure for the product
  6. How can I test the communication between meters?
    We strongly recommend buying our test package if you are doing any testing with Modbus and Autometers products, the price is only £150 pounds and it can save you a lot of time. The package consists of, PC software package program (on disc) and RS 485 Converter and lead.

    The software package shows you how to send a command together with the request string, you will then get a reply from the meter (a response string) showing you how you must construct your software to read the meters successfully.

    Part Number: AMS SOFTWARE and RS485 Mev Converter
  7. What type of cable should I use?
    You should use Beldon 9841 (2core) or Beldon 9842 (4 core)
  8. What should I set the protocol settings to when using an Autometers product?
    The standard protocol setting for Autometers products are 2 wires, baud rate 9600, RTU, Floating point, High word first and parity even. The products have been designed so you can adjust to new settings such as different baud rates and different speeds this information is available in the hand book provided with the product.

Trouble shooting

  1. I can not see any meters when connected
    a) Check to see if the meters you are trying to monitor have power connected to them.
    b) Check the power is connected to the Horizon or management system you have
    c) Check the cables are connected correctly and the terminators are connected to both ends of the LAN.
    d) Check all the meters have a modbus address set.
  2. I can only see some of the meters, not all of them
    a) Check that no modbus addresses are the same they must all be unique
    b) Check that the terminators are at the begging and the end of the LAN, if they are in the middle of the LAN you will see only the meters up to were you have connected the terminator resistor.

Helpful information

If you are having difficulties in reading all or some of the meters connected to the LAN, start by connecting the first one and prove that you can communicate properly. Then keep adding one meter at a time until you can read all the meters satisfactorily.

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Electricity Meters and Power Monitoring Systems
4b Albany Road, Chorlton-cum-Hardy, Manchester M21 0AW
Tel: +44 (0)161 861 9056 / Fax: +44 (0)161 881 3745