GE B90 UR Series Instruction Manual

Also see for B90 UR Series: Instruction manual

Contents

GE Multilin B90 Low Impedance Bus Differential System B-1APPENDIX B B.1 MODBUS RTU PROTOCOLBAPPENDIX B MODBUS COMMUNICATIONSB.1MODBUS RTU PROTOCOL B.1.1 INTRODUCTIONThe UR-series relays support a number of communications protocols to allow connection to equipment such as personalcomputers, RTUs, SCADA masters, and programmable logic controllers. The Modicon Modbus RTU protocol is the mostbasic protocol supported by the UR. Modbus is available via RS232 or RS485 serial links or via ethernet (using the Mod-bus/TCP specification). The following description is intended primarily for users who wish to develop their own master com-munication drivers and applies to the serial Modbus RTU protocol. Note that:• The UR always acts as a slave device, meaning that it never initiates communications; it only listens and responds torequests issued by a master computer.• For Modbus, a subset of the Remote Terminal Unit (RTU) protocol format is supported that allows extensive monitor-ing, programming, and control functions using read and write register commands.B.1.2 PHYSICAL LAYERThe Modbus RTU protocol is hardware-independent so that the physical layer can be any of a variety of standard hardwareconfigurations including RS232 and RS485. The relay includes a faceplate (front panel) RS232 port and two rear terminalcommunications ports that may be configured as RS485, fiber optic, 10Base-T, or 10Base-F. Data flow is half-duplex in allconfigurations. See chapter 3 for details on communications wiring.Each data byte is transmitted in an asynchronous format consisting of 1 start bit, 8 data bits, 1 stop bit, and possibly 1 paritybit. This produces a 10 or 11 bit data frame. This can be important for transmission through modems at high bit rates (11 bitdata frames are not supported by many modems at baud rates greater than 300).The baud rate and parity are independently programmable for each communications port. Baud rates of 300, 1200, 2400,4800, 9600, 14400, 19200, 28800, 33600, 38400, 57600, or 115200 bps are available. Even, odd, and no parity are avail-able. Refer to the Communications section of chapter 5 for further details.The master device in any system must know the address of the slave device with which it is to communicate. The relay willnot act on a request from a master if the address in the request does not match the relay’s slave address (unless theaddress is the broadcast address – see below).A single setting selects the slave address used for all ports, with the exception that for the faceplate port, the relay willaccept any address when the Modbus RTU protocol is used.B.1.3 DATA LINK LAYERCommunications takes place in packets which are groups of asynchronously framed byte data. The master transmits apacket to the slave and the slave responds with a packet. The end of a packet is marked by dead-time on the communica-tions line. The following describes general format for both transmit and receive packets. For exact details on packet format-ting, refer to subsequent sections describing each function code.• SLAVE ADDRESS: This is the address of the slave device that is intended to receive the packet sent by the masterand to perform the desired action. Each slave device on a communications bus must have a unique address to preventbus contention. All of the relay’s ports have the same address which is programmable from 1 to 254; see chapter 5 fordetails. Only the addressed slave will respond to a packet that starts with its address. Note that the faceplate port is anexception to this rule; it will act on a message containing any slave address.A master transmit packet with slave address 0 indicates a broadcast command. All slaves on the communication linktake action based on the packet, but none respond to the master. Broadcast mode is only recognized when associatedwith function code 05h. For any other function code, a packet with broadcast mode slave address 0 will be ignored.Table B–1: MODBUS PACKET FORMATDESCRIPTION SIZESLAVE ADDRESS 1 byteFUNCTION CODE 1 byteDATA N bytesCRC 2 bytesDEAD TIME 3.5 bytes transmission time