****** DeviceNet De-mystified... The Basics ===================================================
DeviceNet has been around for just over a decade. It extends back to a time I like to refer to as 'the war days'... that is the bus war days.
In the early-mid 1990's most PLC manufacturers were trying to develop the 'standard bus'. Naturally this can only lead to one thing for the industry. (ie. a mess and no real standard) Well, guess what we got folks?? That's right, a mess and no real standard. But I'm sure no vendor would say that 'on the record' because their bus IS the standard... as long as you're using their PLC that is.
DeviceNet is one of the many 'fieldbuses' available on the market today. Fieldbus is a big techno-word that basically means a network that is optimized to exchange data between small devices and a main larger device(s). The devices send/receive status and command messages to each other. It's main purpose is wire saving. We'll explain more details later on... and see why wire savings is a HUGE benefit.
The early days of DeviceNet were the Allen-Bradley only days. But in
1992 they started to share information with the automation community and even invited competitors to become members of their DeviceNet community. At the March 1994 ICEE show in Chicago DeviceNet was released to the automation world and in 1995 Allen-Bradley 'surrendered' DeviceNet to the ODVA (ie. Open Device Vendors
Association) It remains an open protocol today and anyone can get the actual specifications... for a $300 fee that is.
DeviceNet is quite smartly based on CAN (ie. Controller Area Network).
Honeywell also has a bus called SDS that is based on CAN as well. (See what I mean about bus wars... everyone has a 'standard bus'. I'm going to call my bus 'school bus'... ohhhh- that's already taken) CAN comes from the automotive industry and was developed by BOSCH in Germany for Mercedes way back in the 1980's. Back then Mercedes and all cars had a 'wiring harness'. (lift the hood of an older car to see the spaghetti wiring of the time...) The wiring harness was essentially the control panel for the car. All of the cars wires went back to here and it was obviously 'chock full' of wires. There had to be a better way of sending all the wires back to here. So, BOSCH engineers developed the 'CAN bus'. Now a single cable could be wired throughout the vehicle and the individual systems (ABS, brake lights, locks, windows, air bag, etc) could just 'tap into' it. The bus also distributed power... so hopefully you can easily see the wire savings.
To see this wire savings benefit in our factory environment, let's look at an example:
DeviceNet allows up to 64 'nodes'(ie. taps in the car example above).
That's actually 63 nodes and 1 brain. Each node has a simple device connected to it.(like a sensor, for example) Each sensor will need power (so that's 1 wire for + and 1 wire for - ) and a signal wire so we can know if the sensor output is on/off. So, each node (device) connected needs 3 wires.
The maximum distance we can go to maintain the fastest speed is 100 meters. We can go 500 meters at the slowest speed but let's use the fastest speed for our example.
We therefore need 3 wires x 100 meters x 63 nodes = 300 x 63 = 18,900 meters of wire! Since 1 meter is about 39 inches, we'd need about 737,100 inches or 61,425 feet of cable... WOW. I want to see you troubleshoot a wiring problem in that application! How would you color code the wires??
By using a bus system like DeviceNet we'd dramatically reduce the number of wires needed. A DeviceNet bus has 5 wires bundled together
(2 for power, 2 for communications signal, and 1 shield). We'd run the bundle the full 100 meters and tap into it near each device to connect the device to the bundle. The taps are generally quick-disconnect connectors so wiring is simple. Now to troubleshoot we'd simply make sure the bus was alive (easy if the devices up the line were working) and then check the individual device where the problem was.
Simple. Neat wiring. Easy to follow. Easy to document. Easy to setup.
Easy to troubleshoot.
Since DeviceNet is based on CAN, which comes from the automotive industry, it has a background of high reliability and speed. (If your air bag doesn't work because of a bus fault or delay that would be a problem... wouldn't it??) The DeviceNet brain (ie. cpu) is also good in tough environments (since they are derived from the automotive
industry) so high temperatures and noise are not a major issue.
Devices such as motor starters, variable frequency drives, HMI displays, barcode readers, sensors, and even limit switches are all available with DeviceNet capability built-in.
Let's now take a look at the specifications of DeviceNet real quick and explain each.
DeviceNet is an 8 byte bus system which translates to the fact that it doesn't send as much data as something like Ethernet so it's good for small device communication (like sensors, motor drives, etc) as opposed to computers and other 'data hungry' devices.
The devices all connect to a single DeviceNet cable via taps into this main cable. (Like branches on a tree all connect to the trunk, for example. The leaves would be the individual devices but wouldn't fall off when the weather starts to get cold like here currently in NY
state!)
Everything 'talks' at speeds of 125Kbps (125kilo-bits per second. Kilo means 1000 so 125Kbps is 125,000 bits of data per second), 250Kbps or 500Kbps. On a thick cable at 125Kbps we can have a maximum trunk (like a tree) cable length of 500 meters. (that's a tallllllll treeeeee!) At 250Kbps the cable can only be 250 meters max. And at the fastest speed of 500Kbps we can have a 100 meter max trunk cable length.
Each tap into the main trunk cable (like a branch on the tree) can be a maximum length of 6 meters regardless of the communication speed. (So the branches on the tree can be 6 meters long maximum) The total of all taps can be a maximum length of 156 meters at 125Kbps, 78 meters at 250Kbps, and 39 meters at 500Kbps. (chop all the branches off the tree trunk and lay them in a line end-to-end and their total cumulative length cannot not be longer than 156 meters... assuming slow speed)
We can draw a maximum current of 8 amps through the system (remember that power is supplied to all devices on the trunk line at 24VDC)
There is also duplicate address detection. Each node supplies an address that cannot be duplicated.(otherwise who are we 'talking' to?
Like if there were 2 Phil Melore's standing next to each other how would we know which we were talking to???) When the node is connected to the bus it announces it's address (ie. name) to the network. If the address (name) already exists on the network, that new device will not be allowed to be put into 'run mode'.
There is error checking done by the CRC method. (Cyclical Redundancy
Check) Ethernet uses it and basically makes sure that the data that was sent is received without error. (ie. You say 'hello' and I receive 'hello' and not 'herra')
Each end of the trunk cable needs a 'terminator'. Basically this means that we add some electrical resistance (121 ohms each end of the cable) to each end of the cable to electrically 'balance' the network.
Finally, just for reference, black/white conductors are specified for communications, red/black for power, and a bare braid for shielding.
Vriti Edustore
