Networking A method of exchanging data between two physically distant nodes. Two devices are said to be networked when a process in one device is able to exchange information with a process in another device.

Why Network? Why network a PLC control system? Less wire and complexity in construction Two way communications Longer distances Multiple Systems supervised by “controller”

HART Protocol Highway Addressable Remote Transducer Communicates over 4-20mA current loops Modulates the digital signal on top of the analogue current signal in the loop Early implementation of Fieldbus

HART Protocol cont.

Frequency Modulation carrier signal

Serial Communications RS 232  Point to Point Protocol  Low line speed (baud rate)  Short Distances ~ 100m  Still use for programming the PLCs

Serial Communications RS 422  Point to Point Protocol and  Multi-drop. 1 Master & 10 receivers  Greater Distances ~ 1500m  Common use is for RS-232 extenders

Serial Communications RS 485  Point to Point, Multi Drop and Multi Point  Up to 32 devices  Uses Differential signalling  Greater Distances ~ 1200m  Only requires 2 wires and configured as a bus

Modbus Developed by Schneider Electric in the late 70’s Has become the de facto standard Has been “open sourced” in that it is openly published and royalty fee

Modbus Can link up to 250 devices on a RS-485 cable Can be encapsulated inside other protocols (Ethernet) with the use of protocol convertors (blackbox convertors) or Modbus TCP frame format Serial based on Master/Slave operation. Ethernet more like a peer-to-peer network

Modbus Client/Server communication on an Ethernet Network MODBUS Request is the message sent on the network by the client to initiate a transaction MODBUS Indication is the request message received on the server side MODBUS Response is the response message sent by the server MODBUS Confirmation is the response message received on the client side

Modbus The Modbus messaging services (client/server model) are used for real time information exchange Between two device applications Between device applications and other devices Between HMI/SCADA applications and devices Between PC and device providing online services

Modbus Typical Modbus network with multiple clients/servers

Modbus Defines simple Protocol Data Unit (PDU) and application Data Unit (ADU)

EtherCAT Open high performance Ethernet-based fieldbus system. Founded in 2003 Germany Uses “standard” Ethernet for the interexchange of data Synchronization with distributed clock Works as a “peer to peer” network

Fieldbus Standardised as IEC Multiple topologies and protocols (types): Star, Ring, Daisy-chain Type 1 Foundation Fieldbus H1 Type 2 ControlNet Type 3 PROFIBUS Type 4 P-Net Type 5 FOUNDATION fieldbus HSE (High Speed Ethernet) Type 6 SwiftNet (a protocol developed for Boeing, since withdrawn) Type 7 WorldFIP Type 8 Interbus

More on Protocols Open vs. closed protocols and standards Advantages and disadvantages: Time taken to develop Cost per license Adoption rate Ability to change and grow

More Modern Protocols Why the need?  Each transducer requires a discrete input (unless TDMA multiplexing)  Distance between controller and transducer only up to ~1500m  More information required than just 1 output allows  More wiring required for each transducer.

OSI Model TCP/IP Transfer Control Protocol Internet Protocol Includes TCP and UDP Requires the use of “ports”

OSI model Data unit LayerFunction Host layers Data 7. ApplicationApplicationNetwork process to application 6. PresentationPresentation Data representation, encryption and decryption, convert machine dependent data to machine independent data 5. SessionSession Interhost communication, managing sessions between applications Segments4. TransportTransportEnd-to-end connections, reliability and flow controlflow control Media layers PacketPacket/DatagramDatagram3. NetworkNetworkPath determination and logical addressinglogical addressing Frame2. Data linkData linkPhysical addressing Bit1. PhysicalPhysicalMedia, signal and binary transmission

Lab Work Connect two laptops up with Null-modem cable, use a terminal emulator program (putty) and configure for serial communications. Try settings baud rate to 100 and note speed. Then connect one laptop to oscilloscope, and observe the various voltages and time scales.

LetterASCII CodeBinaryLetterASCII CodeBinary a A b B c C d D e E f F g G h H i I j J k K l L m M n N o O p P q Q r R s S t T u U v V w W x X y Y z Z