I have been asked a question....which I don't know the answer to, hopefully someone here will have come across this before (I can't be the first).

I have a PLC that begins using Modbus Serial, then this gets converted/encapsulated into Modbus TCP/IP. I know this works in industry, so that part is fine.

What I want to know is, if I then encapsulate Modbus TCP/IP into HTTPS, should the end result still be achievable (i.e. reverse the process so the information can be read)?

Many thanks for any help.
Liam

 

You would need a sort of gateway that converts "http get" requests to modbus requests.

This can be done with:
http://en.wikipedia.org/wiki/SOAP

If you want a standard means use OPC XML-DA
http://www.dopc.kassl.de/opc-server-gateway.shtml

 

Yes, its definitely achievable. There are several commercial products available that provide a web based HTML interface to PLCs.

But from your question and requirement for HTTPS I am guessing that you want a more direct interface using HTTPS to serve an XML data stream to another data system somewhere.

The major issue you face is that Modbus Serial and TCP are typically Master/Slave communication messages sent on a cyclic - i.e. continuous - basis. (Yes, we know that modbus comms doesn't need to be cyclic but it typically is) So you have a continuous stream of messages coming from your PLC slave in a format which contains lots of information in a very rigid structure. You then want to translate this into a Client/Server communication where your client(s) may or may not request that data at any arbitrary time, using XML which has a pretty arbitrary data structure.

So you are probably going to require the following components ....

* An off-the-shelf Modbus TCP / Serial interface to the PLC

* A database to store the information read by the PLC interface until it is requested by the HTTPS client.

* A web server to serve the data from the database to your clients using HTTPS.

If you're able to use open source software then you could do this fairly easily with a MySQL database, a Windows based IIS or Linux Apache web server and a pretty simple PhP web page application. I found a ModBus to SQL application with a couple of minutes searching using google here -> http://sourceforge.net/projects/modbus2sql/ which looks like it could provide a good starting point.

Rob
www[.]lymac.co.nz