In part 1 we did a quick example of reading and writing via Modbus.
That’s great, but there are several things you need to be aware of when using Modbus
to communicate with the ACN Family of drives.
You can use this list of topics as an index to fast forward to any one in particular.
So why is the Ground wire so important when using RS485?
After all it’s just a differential signal pair so all we care about is the difference
between the two signals – not how far they are away from ground right?
The beauty of differential signaling is if you have the same noise on both signals, it
cancels when you take the difference between the two signals which results in a nice clean
signal the drive can use.
And in theory, that’s great.
But .. in the real world things aren’t so clean.
The issue is, the electronics in the drive can only handle so much voltage coming in.
More than that and the coms interface stops working or even possibly gets fried.
In a factory, two devices trying to communicate with each other often have different reference
grounds – especially when the devices are separated by some distance or even in different
buildings.
So when the signal hits the other device, it can come in at a level above the max the
device’s electronics can handle and the electronics don’t work or even possibly
burn up.
Providing a signaling ground wire ensures that both ends are using signaling relative
to the SAME ground which prevents the electronics from being exposed to dangerous voltage levels
and gives the drive a good clean reliable signal to work with.
You will save yourself a lot of headaches and frustration if you use a good low capacitance
cable that’s rated for signaling.
I used this cable from AutomationDirect which is specifically designed for data communications.
But, did you notice it’s a 4 wire cable?
We only needed one pair of signal wires and a ground.
Why did I use a 4-wire cable?
It’s easier to keep a 4-wire cable in stock because it can be used for many other purposes
and it’s basically the same cost as a three wire cable.
I only used one of the spare wires for the ground wire here in my office, but you could
certainly use both of the extra wires to give you an even better ground path – especially
over longer runs.
And while we are on the subject, make sure you connect the shield to chassis or earth
ground at one end only.
Why?
Because if you ground both ends you now have a closed circuit which allows current to flow.
So when other nearby cables or electromagnetic noise sources impose magnetic fields on the
shield, it creates current flow.
The current flow in the shield then creates a magnetic field inside the cable which creates
extra current flow in your data wires which adds noise to your data signals.
If you leave one end of the shield disconnected, you have an open circuit so current can’t
flow down the shield and create that interference inside the cable.
The ACN drive provides a convenient chassis ground here.
Why did we use 9600 baud in the previous video?
Because slower baud rates give you better noise immunity.
So in a noisy environment like a factory, always use the lowest baud rate you can get
away with.
It’ll save you a lot of headaches later on …
And of course, when setting up your RS485 network, be sure to add a termination resistor
at the end.
That prevents energy from being reflected back up the cable and messing with your signals.
On the ACN drive, you can enable a termination resistor using this guy right here tucked
up under the cover.
This handy diagram also shows the location.
You can use digital outputs to signal when coms have been lost.
That can be real handy when you are trying to debug issues or just want to give your
controller some feedback that coms are working.
On the base drive you would set either the output relay [OU.31] to a 13 or the digital
output [OU.33] to a 13.
The output will change when there is a loss of command from the RS485 coms.
How does the drive sense a loss of Modbus Commands?
And if it has lost coms, what should the drive do?
Easy.
Just setup your controller or PLC to send commands via Modbus RTU … on a periodic
basis.
Maybe poll a set of parameters you want to monitor on a regular interval.
Then set Parameter Pr.13 to the maximum time you want to allow for each periodic command.
If the drive doesn’t get a command within that amount of time, then it will toggle the
output we just talked about, and then it will do whatever you put in Parameter PR.12.
You can tell the drive to do nothing, free spin down, use the normal deceleration, keep
running at the last known input, keep running at the last known output level or go to the
frequency you specify in parameter Pr.14.
Did you notice in the previous video that Modbus TCP and EtherNet/IP use different addressing
than Modbus RTU?
So be careful if you ever decide to switch to Ethernet Coms – you have to add a one
to each address for everything to work.
If you are writing a group of parameters that includes the drive frequency, make sure you
also specify the corresponding RPM.
If you forget to update the RPM then when this group is written the RPM parameter will
overwrite the frequency.
So make sure they are the same when sending a group of parameters that contains either.
As a reminder, to calculate RPM just take the frequency, multiply by 60 to convert it
to minutes, times 2 for the positive and negative pulses, and divide by the number of poles.
For example, 30 Hz on a 4 pole 1800 RPM motor is 900 RPM.
Exactly what we expect.
In the previous video we used an edge triggered contact to issue the group read.
That was just to keep the video simple.
In a real world application, you would normally setup some kind of timed loop so the group
can be updated periodically.
That way you don’t have to remember to manually update the command each time you want a new
set of values.
The ACN Family of drives have virtual digital inputs.
Which means coms can do pretty much anything you would normally do from a digital input
terminal.
For example, if you normally have a digital input controlling the run/stop of the drive,
you could assign that to a virtual input and then toggle that virtual input via coms instead
of using the command we used in the previous video.
Just another way to do the same thing.
Click here to learn more about the ACN Family of drives.
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