1.7 BUS TYPES
- Smart Distributed System (SDS)
• Devicenet is meant as a noise resistant, robust, shop floor network.
• The main change that the user will notice is that a rack of a PLC can be broken up and CPU, I/O, etc. are now connected by Devicenet.
• Devicenet is an open standard, so components can come from a variety of suppliers.
• Based on the CANbus (Controller Area Network) protocol.
- open system is not tied to a single vendor
- diagnosis of devices is possible
- wiring becomes easier (no long wire runs)
- complex devices are easy to connect
- possibility of using a PC based control package
- multiple controllers can share the net
- up to 64 nodes on the network
- lengths of 500m/250m/100m for speeds of 125kbps/250kbps/500kbps respectively
- data packet size of 0-8 bytes
- power and signal in the same cable
- addressing includes peer-to-peer, multicast, master/slave, polling or change of state
- devices can be added/removed while power is on
• Devicenet is a layer 7 - application protocol (Note that CANbus is level 2)
• A typical structure for Devicenet is shown below,
• There are two types of wire used,
Thick trunk - carries up to 8A for power up to 500m
Thin trunk - up to 3A for power up to 100m
• Power supplies are directly connected to the cable to supply power.
• The drop lengths are limited
- total length/speed 156m/78m/39m are 125Kbps/250Kbps/500Kbps respectively
- a single drop is limited to 6m
• The wiring can come in a form of end/connectors,
• Devices may have a red/green status light for the network status.
• Devicenet used the CANbus indentifier to specify,
node number (6 bits) up to 64 nodes on the net
• On a PLC-5 rack you would need a devicenet interface and could use the instructions below.
• On a Softlogix PLC the I/O will be copied into blocks of integer memory. These blocks are selected by the user in the setup of the software. Here the inputs are copied into N9 integer memory, and the outputs are set by copying the N10 block of memory back to the outputs.
• The CANbus (Controller Area Network) uses bit wise resolution for collisions on the network (i.e., the lower the network identifier, the higher the priority for sending)
• A data frame for the CANbus is shown below,
• Each device listens to the frame it sent to ensure proper communication.
• This is intended for communication between controllers, and permits more complex messages than Devicenet.
• A control network that is popular overseas, and primarily pushed by Siemens.
- A token passing between up to three masters
- length from 9600m/9.6Kbps with 7 repeaters to 500m/12Mbps with 4 repeaters
- power needed at each station
- 2048 bits of I/O per network frame
1.7.6 Proprietary Networks
22.214.171.124 - Data Highway
• Major features of the DH+ network,
- peer to peer with token passing
- uses shielded twisted pair cables
- data rates of 57.6Kbps, 230 Kbps
• The basic data frame looks like,
• The data packet looks like that shown below (but it will vary depending upon the requested command).
• Some command examples include,
• Example for the Allen-Bradley PLC-5 Data Highway Network Connection