Series 2000 Communications Handbook

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MODBUS and JBUS ProtocolSeries 2000 Communications Handbook3. CHAPTER 3 MODBUS AND JBUS PROTOCOLThis chapter introduces the principles of the MODBUS and JBUS communication protocols.Note that in the Series 2000, these two protocols are identical, and both will be referred to asMODBUS for the descriptions that follow.3.1. PROTOCOL BASICSA data communication protocol defines the rules and structure of messages used by alldevices on a network for data exchange. This protocol also defines the orderly exchange ofmessages, and the detection of errors.MODBUS defines a digital communication network to have only one MASTER and one ormore SLAVE devices. Either a single or multi-drop network is possible. The two types ofcommunications networks are illustrated in the diagram below;5KPING 5GTKCN .KPM/WNVK &TQR 5GTKCN .KPMJBUS MasterJBUS MasterTXRXTXRXvRX^RS232TXvRX^TX^vRS485vRX^TXJBUS Slave 1JBUS Slave 1JBUS Slave NA typical transaction will consist of a request sent from the master followed by a responsefrom the slave.The message in either direction will consist of the following information;Device Address Function Code Data Error Check Data End of Transmission• Each slave has a unique 'device address'• The device address 0 is a special case and is used for messages broadcast to allslaves. This is restricted to parameter write operations.• Series 2000 supports a subset of Modbus function codes.• The data will include instrument parameters referenced by a 'parameter address'3-2 Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00
Series 2000 Communications HandbookMODBUS and JBUS Protocol• Sending a communication with a unique device address will cause only the devicewith that address to respond. That device will check for errors, perform the requested taskand then reply with its own address, data and a check sum.• Sending a communication with the device address '0' is a broadcast communicationthat will send information to all devices on the network. Each will perform the requiredaction but will not transmit a reply.Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00 3-3
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Series 2000 Communications Handbook

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