topic Re: Modbus Read on Demand in DASYLab

Modbus Read on Demand

COIEngineer — Wed, 10 Sep 2014 17:41:46 GMT

Is there a way to only read a modbus on demand? I want to be able to click a button to "Get data from controller".

Also related, I would like to only write to it on command. I currently have a trigger/relay for the write data, but I cannot tell if modbus is still sending the write command (using the previous line value) or not.

I am currently reading test data through the modbus. It appears as though the extra commands limit the sample rate, leading to missed data. If I only read the data registers, the data has no repeats. With the other commands, I get up to 10 blocks of the same value.

Thank you.

Re: Modbus Read on Demand

TomRizzo — Wed, 10 Sep 2014 21:45:04 GMT

Which device are you try to communicate to?

ModBus, to my understanding, is a continuous process method, so, the only way I know to read or write on command, is like you just said, with a relay and trigger.

If you want further analysis, I would suggest to use an OPC server and use that instead, this module comes with all kind of monitor tools that you can use to troubleshoot as well.

DASYLab supports OPC perfectly.

Re: Modbus Read on Demand

COIEngineer — Wed, 10 Sep 2014 21:54:41 GMT

I'm communicating with an Delta Motion RMC150E Motor Controller. How do you set a read up with a trigger? I don't see any sort of input or action item.

I'm unfamiliar with OPC servers, so I'll have to look into that.

Thank you!

Re: Modbus Read on Demand

TomRizzo — Wed, 10 Sep 2014 21:58:07 GMT

You take the input module into a relay with block data option. Only when you trigger the control output, you will be allowing flow from the module.

This mean thagt unless you trigger the relay, the module will not process and ask for value

Re: Modbus Read on Demand

COIEngineer — Wed, 10 Sep 2014 21:59:55 GMT

Thank you! One last question, does the downstream "block" work on each channel individually, or the module as a whole?

Re: Modbus Read on Demand

JackSprat — Thu, 11 Sep 2014 11:26:37 GMT

Hi all.

I am using ModBus to talk to six PID controllers via RS485.

I use a USB to RS485 converter but the devices appear on a single regular RS233 ( Comm port ) channel.

Now, there is a repetition frequency parameter in the ModBus dasyLab module.

It defaults to 1 Hz when you begin.

I think the ModBus channel just ticks away at the set frequency and sends or receives values regardless

of other DasyLab clocks or timebases.

On a send channel it will just transmit whatever is seen on the module input at the 'tick' time.

Likewise values return at the set rate - whether you read them or not.

Hope this helps.

JackSprat

Re: Modbus Read on Demand

TomRizzo — Thu, 11 Sep 2014 11:50:37 GMT

The whole module will be blocked

@COIEngineer wrote:

Thank you! One last question, does the downstream "block" work on each channel individually, or the module as a whole?

Re: Modbus Read on Demand

TomRizzo — Thu, 11 Sep 2014 11:53:01 GMT

I also have to say that for every project we work that requires any kind of modbus comunication, we just decided to use OPC servers because it is a better way to control devices and DASYLab does an excellent job comunicating with OPCs