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Barry

Maximizing Modbus masters using the UAC-02RSC

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Good morning everyone!

We are about to begin work on a Modbus deployment that will need far more than just the 255 addresses available on the built in CPU master.  I know we can add up to three of the UAC-02RSC UniComm modules for three more masters.  Unfortunately, the masters on the 02RSC have a limit of 512 operations each, so we’re going to need all three modules and then some.

I’m interested in using the serial port on the top of those modules as well.  Unilogic and the built-in help file shows that we can turn them into Modbus masters, but we’re not having any success getting it to communicate with our sensors and devices.  We’re using a small header-to-terminal block plug to give us access to pins two and three (TX and RX) but I’m not sure that’s correct for what we’re doing here.

Thank you in advance for any help you can give us on the serial connection!

61DXBPWfqCL._SX425_.jpg

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So let me get this straight; you're going to poll 256+ modbus items over 3 serials ports that will have 512+ operations over a 232/485 serial port?

In the standard MODBUS network there is one MASTER and up to 247 slaves.  

I would not want to be in the shoes of the startup engineer on this project.  Yes, it can be done.   Yes, you may not be up against the serial limitations, however you're going to have a troubleshooting nightmare if just one of those devices on causes a conflict you're in a fun week or two or six...   

Enough of that.

Let's start at the beginning:   Read this and see if that fills in any gaps you may have missed.

Modbus Over Serial Line V1.02

Are you using a MODBUS Gateway at all?

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Yes, Modbus spec is limited to 247 addresses but Unilogic will let you use the full binary 1-255.  I've never actually connected more than 247 devices so I don't know what would happen. 

Those shoes you're talking about are mine, and I've never had a problem walking in them.  You're only going to end up with conflicts if you screw up the addressing. So far I've managed to count into the mid 200's without hurting myself. Uniapps alone would clue you in to which devices are in conflict if there is one, let alone any basic in-program diagnostics. 

The built-in master is running about 200 small sensors (Dwyer temp/humidity/co2 devices). Each only has 4-6 operations. It's lightweight data-wise; just a long string of devices. Low polling rates, updates only once every few seconds. We've set these networks up many times before with no problems. Surprisingly durable even when the network topology is abused. 

The reason we need such a heavy operations load is our I/O network is comprised of a fleet of Click PLCs (small, cheap, durable, modular devices from Automation Direct).  Unfortunately dollar-per-I/O it's a better deal than the local or remote expansion kits, or even the EX-RC1.  ((EDIT:  We use the Click PLCs because if they lost communications to the Unistream they can act autonomously)) 

Each one handles a ton of environmental and facility control systems.  Easily 200 operations each.  The max for the 02RSC is 512 operations.  While it means we're burning off an entire modbus master on effectively two or three slaves I don't know another way around it.  Also, many of these devices would end up in completely different parts of the facility and having seperate networks would simplify the wiring.

There are no gateways, so far everything has been a direct connection back to the unistream.

So the original question:    How are the 232 ports, when configured in Unilogic as Modbus masters, different from the 485 port?  I'm guessing the hardware inside the module isn't strictly speaking a serial comm port (5v-15v logic, single ended, etc) but rather it's emulating those characteristics.

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Uhmmmmm.  Don't forget only "Up to 32" nodes on the 2RSC.

I'd actually look at having other plcs acting as "sub-masters", that then report their results to the Master via another protocol.  Effectively cascade it all.  This might actually make things easier to implement.

Just a thought.

cheers, Aus

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We would only end up with 2 or three devices on each of the expansion module masters, so the 32 address limit isn't a big deal.  Because those devices have almost 200 operations each we'd max out that limit almost immediately. Hence the need for using the serial ports as additional masters. The idea is to gain another 512 operations; not necessarily more addresses. 

Unfortunately the small PLCs we use aren't capable of upstream and downstream networks, otherwise they would make for a perfect bridge. But for their cost I can't complain.  We might need to consider switching to ones that allow us that method of expansion. 

 

For now utilizing the 232 ports as modbus would solve our problem. Presuming we can get them to work. 

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OK, but now I'm confused because in your first post you say

7 hours ago, Barry said:

need far more than the 256 addresses available

And I wasn't meaning change all your little plcs to a different type, just have a few dedicated different units acting as the intermediarys.  They run through their polling, collate the results into something far more usable and concise, and then send that to the master when requested.

cheers, Aus

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You're absolutely correct, I missed the mark in the original post. We're likely to end up with more addresses than the built-in can handle eventually, but the critical issue is the operations limit on the modules. 

Ah, use one as a hub for expanding the network. That or a gateway.  Good call; that's likely our best bet. ((Edit: you're actually more right than you realize. It might be time for us to find a stronger side kick than the existing little PLCs)) 

If we could use those serial ports we'd be able to pull it off with just existing hardware. Our last install (half this size) was already a massive system... If we can stick to snap-on or built in systems it would be nice for a change. The last thing we need is ANOTHER panel when we can squeeze what we need out of what's there. 

2018-12-20 20.15.03.jpg

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23 hours ago, Ausman said:

Don't forget only "Up to 32" nodes on the 2RSC.

  You're right, I had an old spec sheet for the 02RSC that says 32 devices, but Unilogic doesn't seem to have a problem with me adding and compiling with triple that number (although it does enforce the 1-247 addressing limit, while the built-in master lets you assign 1-255).  I'm making a stupidly-dangerous assumption that if it compiles it might allow me to operate that way... I have a feeling I'm missing a lot of details on these ports.  Wish there was better documentation on them.

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Well...you could always do a test of more than 32 on your testbench and see if it works!  But in this case I think it will be correct.

However, I get the feeling that what you are essentially doing is a BAS.  Futher to my suggestions, you could retain all your current infrastructure by having my "sub-masters" plcs on each building level/major location, gathering data via the existing 485 network on that level, then holding that collated data until read by the Unistream via another 485 port on the subs.  This way you would not need significant changes to the exisiting wiring topology.  It would almost be a case of a few cuts and splices on each building level.  (And it sounds like a bit of a hodgepodge wiring setup anyway, with the topology being "non-standard"!  🤣  It is surprising how tolerant 485 can be at times.  I've seen all sorts of odd connections that work.  And I've also seen the ones that are perfect and have issues!  Go figure. )

The only real difference would be the need to have the collating plcs having 2 x 485 ports, the ability to poll all the other the little plcs on that level and collate that data ready for transmission to Earth Central.

Edit PS.....And something else is a Winner, you lose the need for so many 02RSCs.  Hmmmm!!!  That might pay for the different Subs!

cheers, Aus

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That's a brilliant point.  The Vision series aren't too expensive, and would allow us to really expand almost infinitely one modbus (borrowing from IP terminology for lack of better word) subnet at a time.      Create a backbone that's effectively...

 Unistream --->  Vision ---> Vision ---> Vision ---> ad infinitum

... and split off from the Visions for our smaller I/O heavy devices.  

 

In a related point I did get a response from Unitronics about the ports on the 02RSC...

"...You absolutely will have the ability to have them both be masters. The RS232 port will be one to one where the RS485 port can be the master to multiple slaves in a daisy chain network...."

 

But that's still at odds with what Unilogic will allow me to configure.  I'll respond to them in the AM, and maybe after some coffee I'll be able to communicate a little more clearly than I usually do.  

Thank you for your help and guidance!

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17 hours ago, Barry said:

That's a brilliant point. 

That's OK.  I'll send you my Xmas invoice for making your systems so much better!  😉

cheers, Aus

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On 12/20/2018 at 6:32 PM, Barry said:

We would only end up with 2 or three devices on each of the expansion module masters, so the 32 address limit isn't a big deal.  Because those devices have almost 200 operations each we'd max out that limit almost immediately. Hence the need for using the serial ports as additional masters. The idea is to gain another 512 operations; not necessarily more addresses. 

Unfortunately the small PLCs we use aren't capable of upstream and downstream networks, otherwise they would make for a perfect bridge. But for their cost I can't complain.  We might need to consider switching to ones that allow us that method of expansion. 

 

For now utilizing the 232 ports as modbus would solve our problem. Presuming we can get them to work. 

Can you explain how many variables are you reading/writing in each operation?

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