Ethernet Commands

Wondered if there were any thoughts on this.

I am developing a project and with alot of package plant on an ethernet ring (S7-300). My original plan was to read data from each PLC and let them read my system and thus use the 'Put' function block.

Against my better judgement one of the packages requires some commands from my system.....I have a nice routine for the reading which polls each PLC in turn, thus reading one at a time.

What I realize is that the commands may have to be sent instantly and during a write the read may have to be stopped.

Does anyone have any advise, tips etc.

YoungWint said:

Wondered if there were any thoughts on this.

I am developing a project and with alot of package plant on an ethernet ring (S7-300). My original plan was to read data from each PLC and let them read my system and thus use the 'Put' function block.

Against my better judgement one of the packages requires some commands from my system.....I have a nice routine for the reading which polls each PLC in turn, thus reading one at a time.

What I realize is that the commands may have to be sent instantly and during a write the read may have to be stopped.

Does anyone have any advise, tips etc.

Click to expand...

Hello YoungWint,

Maybe I have a tip. What is helpful is making a boolean that is always true(or clock memory bit) on the PLC. When you want to get data from the PLC you see directly if the PLC is online and able to send data.

kind regards,

MvdVen

To 'send instantly' is a challenge: with typical peer-to-peer IP communications (like you get with PUT), you will send to each device separately. If you need to send to a bunch of clients simultaneously, take a look at the Siemens UDP multicast capabilities. That will allow you to send data to a group of recipients simultaneously.

Regarding the read data, rather than having a 'master' PLC that polls for data from all slaves, I tend to use the PUT commands to send the data from each 'slave' PLC up to a 'master' PLC. If you will, I use a 'blow' architecture rather than a 'suck' architecture. The Siemens PLCs handle receiving from multiple clients to one PLC (PUT from multiple PLCs) well. I'm very careful to limit the rate of the sending PUTs to no more often than once per second to avoid overloading the receiver: the advantage of the 'blow' vs 'suck' is that latency is very fast (the sending PLC reports its changes immediately) and I found the resulting code very easy to understand and debug on all ends.

When you say 'during a write the read may have stopped', I don't know what you mean. The Siemens PLCs can handle multiple read/write operations simultaneously (the number depends on the PLC) so I will typically have a couple asynchronous comms going at the same time. Its definitely a bit tricky to configure: lots of testing is how I eventually figured it out.

I got this going by picking the brain of my local Siemens apps engineer and reading the manual comms section over and over. Its in the Siemens documentation but was difficult reading for me.

I was meaning can you get and put at the same time thus when putting would have to inhibit getting.

I have set up a block that read one plc at a time by indexing through. As this block is already developed I think I will create a second block for putting the commands to the plcs that require this function and base it on a similar methodology!

Well thats a starter for 10 anyway.