modbus serial or tcp?

[vijay shankar]

Dear all,

I have a job to write a labview program to communicate with a TEC controller..

There is a RS232/RS485 port in the controller.

I just have once controller in the lab and I need to develop this program in a system with XP OS(a fairly new system. just to say that its not that old & has usb support)

Now I have two problems for which I need to find the answer.

1. The controller's manual defines how to communicate with it using RTU.

     Now, I am not sure of this, which should I use for using Modbus protocol to communicate with the device

* TCP

* Serial (I suppose this one coz I need to use a serial port. Please help me decide) 

2. How to connect the controller to my system if I need to use Modbus for communication?

     Computer <==========> [???]<===========> controller with RS232/RS485 port

      Will any USB-RS232 cable be sufficient to connect to the computer provided I use Modbus?

Or if I need only RS232 serial cable to connect, where do I connect it to my system. Do I need to buy any PCI card? If yes any specification I have to look for?

 As I am not an electronics student and Computer science is my major, I dont have fair enough idea reg this. 🙂 and am new to labview too..

Appreciate any help. Would appreciate any extra tips if you have experience in this by anychance 🙂

Thank you!! 

[FakStory]

Hi there,

                I spend a lot of time using the RS-232 serial port. This is easy to use with the VISA example of labview. By the way, TCP communication is kindly the same things than a serial one,(either with VISA communication, but allow you to "name" your instrument with an IP address. For a small Network, this is a good idea to talk with many instruments.

You can use a USB/serial, but you may have to check the "NIVISA tulib.dll NIVSApassport for TULIP" in Measurement and Automation program...

, in Tools, NI-VISA, VISA Options,,,

Then, in the new windows...,

Passport, then check "NIVISA tulib.dll NIVSApassport for TULIP"

It would allow labview to use your (unNI) USB/Serial with NI.

Try find some driver in Labview

in...Tools, INSTRUMENT, INSTRUMENT DRIVER...Then Follow the instructions...

Use those sub-vi to communicate with your device...

By the way it may be better to use Serial (RS-232 serial port) if this is only for one instrument...locally...

hope this help

Fakstory

[vijay shankar]

Hi ,

I did not have a menu called NI-VISA in the MAX program.

May be bcoz I currently have evaluation version in my laptop, but use the full version in my college. So will check on that and see if i can check that option.

Which will be the cheaper option. via RS232 or USB adaptor..

If using a RS232 cable, do I need a PCI card. If so, what will be the specification of the PCI card? 

Note: Am just controlling a single controller device.

Thanks for you help..

Kudos to you!! 🙂 

[Dennis_Knutson]

You asked many of the same questions and got answers in this thread. I don't understand why you are asking them again and I don't know why you are mentioning TCP now. If your instrument does not have an ethernet connection, why are you asking about TCP? 

If your laptop/pc has a serial connection, then you do not need anything more than the correct cable to connect to an RS-232 connection on the instrument. If the instrument requires RS-485, then you need a converter. If your laptop/pc does not have a serial port connection, then you will need to buy either a USB or pci card to communicate to either RS232 or RS485. Obviously, a USB device can be used with either and obviously, you would want the correct type (either RS232 or RS485). You keep talking about RS232/RS485 as if they are the same thing. They are similar but not identical. You have to know which one the instrument has.

If you don't have NI-VISA on your laptop, it's because you did not install it.

I also don't understand the previous answer about tulip support. It is not at all relevant and not at all necessary for support of a USB-RS232 converter.

[FakStory]

I got some problem with an usb/GPIB adapter last year and this is the tulip.dll who solved it...Of course it wasn't a USB/SERIAL adapter...

FAK

[vijay shankar]

Dennis,

Thanks for answering in the previous thread..

but the reason why i posted this question of usb or on board rs232 port is coz, I read in few forums that using USB to rs232 adapter for MODBUS RTU protocol, has some issues arising due to the delay caused by the USB adapter. This actually is bcoz of the silence caused by the USB driver and so ppl have adviced not to use USB adapter in case we use MODBUS RTU.

BTW, it is ok if I use MODBUS ASCII protocol to communicate.

As I read this in some forum which is not that active as this is( with ppl like you giving constant support and help to ppl like me, I just wanted to confirm it).

Here is one such webpage which talks about this issue    http://www.control.com/1026212927/index_html

I am sorry to have asked the question again, but I wanted any MODBUS expert if any to give me a suggestion as this is going to be a critical decision on my part.

Thanks for you continued support.:)