LabVIEW connection to Password Protected Access Point with Modbus

[stacey54]

Hello.  I'm trying to use LabVIEW to control a wireless access point via Modbus.  The access point is password protected.  How do I enter a password to initiate communication?  I have looked at both the vi method and the i/o server method, but I have not been able to initiate communication.  I can use a hyperterminal to set up the access point characteristics, after entering the ID and password.  Any help or examples would be appreciated.

[cbutcher]

How do you send identification credentials in Hyperterminal, or similar? You should be able to do the same with LabVIEW. It might be that there is no existing VI as a default part of LabVIEW that sends the appropriate commands or strings, but if needed you can code this using more low-level components and just send that part of your communications before you begin with your other VIs.

[edjones93]

Hi,

Could you provide the model for the access point so we can find the manuals?

Could you also elaborate further on the behaviour you currently experience, are you only ever able to communicate via hyperterminal? Or can you grant access via hyperterminal and then use Modbus?

What makes you think Modbus would work?

As far as I am aware Modbus does not have a credentials system nor does it store sessions, it is a basic RS485/TCP communication method which essentially breaks down into bytes.

http://www.simplymodbus.ca/FAQ.htm

Is a good resource for understanding the protocol, I would contact the vendor and inquire as to whether this is possible over Modbus. What commands do you send via hyperterminal to make this work? You should be able to achieve the exact same through VISA.

Best regards,

Ed

[stacey54]

I'm using the GE MDS NETioA access point, link is below.  It communicates via Modbus TCP.  I've included screenshots.

http://www.gegridsolutions.com/Communications/catalog/NETio.htm

I connect to the hyperterminal via PUTTY, Telnet by entering the IP address.  I am then asked for a login ID and password.  From there I get an information screen and the option to go to the main menu for configuring the device.

I have tried using the Plasmionique example to establish communication.  Initially, I can open the port, however, I get an error when I try to read or write.  

[JÞB]

You are a bit confussed.  the Hyperterm is opening the device UART (COM1) as a serial port.  That would use a straight-up VISA:Instr I/O Session not MODBus.  the MODBus protocol is for the Payload Port (COM2) and passes the data to-from your app over the wireless connection to the endpoint.

[rolfk]

Your mixing and mashing protocols together that don't really have anything to do together.

In Putty you use the Telnet protocol to create an unsecured connection to the build in shell of the device.

With Modbus TCP you connect to the Modbus implementation inside the device, which is a completely different protocol. 

Telnet allows you to communicate with the device in a text based command language that is interpreted by the shell parser on that device.

There might be a chance that the Modbus TCP implementation expects a parallel authenticated Telnet connection from the same IP address in order to accept a connection. While this provide a minimum amount of security from accessing the Modbus interface maliciously, it is a pretty bad security implementation as it could be easily circumvented by a man in the middle attack.

But are you even sure the device has any registers defined at register address 0? That is not very common for many Modbus devices. And before that you should definitely change the port 23 in the Modbus example to something more meaningful. Port 23 is used for Telnet and a Telnet client for sure will not understand any Modbus commands, no matter if you use the Modbus ASCI or Modbus RTU variant.

So far I'm not convinced that the Telnet connection has anything to do with the Modbus connection. But since they require registration in order to provide their valuable manuals to anyone, I can't look at the manual.

[JÞB]

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@rolfk wrote:

So far I'm not convinced that the Telnet connection has anything to do with the Modbus connection. But since they require registration in order to provide their valuable manuals to anyone, I can't look at the manual.

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I'm Registered

[rolfk]

Well, while the documentation is a bit vague in some parts, it does state that the device only supports Modbus RTU protocol as a slave device, which is quite common for the device side. I can't quickly find any mentioning of what TCP/IP port it would use, but standard for Modbus TCP is 502. 

[stacey54]

Update:  I assumed that the error was security related, but it was not.  It is an issue with hardware and I'm waiting for additional equipment to arrive before I can get the system fully configured for communication. Thanks for your responses.