I'm trying to understand the most optimal way to communicate with Ignition and a Siemens 1511 CPU.
I've been reading on various forums and it seems that OPC UA is preferred over Ignition's Siemens driver. However, it feels like it's very easy to hit the maximum limit of 1000 tags. For example, I have a DB for an FB for analog inputs, as shown in the image.
Roughly calculating, this DB contains a total of about 40 variables. This means that the maximum limit of 1000 tags is reached by creating just 25 of these Analog DB types (1000/40 = 25). This is for just one object type; then add pumps, inverters, valves, and so on.
The image shows variables I want to write/read from Ignition to the Siemens PLC for a level transmitter.
It's easy to find examples on YouTube with just a few tags being exchanged between Ignition.
Does anyone here have experience with a larger Ignition + Siemens PLC project who is willing to share their experiences?
The issue with all this is not the tag limit, it is what the tag limit represents.
The tag limit represents a limit where Siemens realised that smaller slower processors will not deal with the overhead of the communications required to provide external access over OPCUA.
OPCUA is a great way to have standard communications with metadata and reliable comms.
It is not an optimised part of any PLC manufacturers firmware, the PLC is optimised to control IO and motion accurately and reliably. Adding thousands of tags to a PLC to be monitored via SCADA is not priority.
The 1511 has 150kb of program memory. That tells you a lot about the raw performance of the CPU, its not designed to be the SCADA centre of the universe.
On the other hand, Siemens has optimised the S7 protocol, which is what it uses for the HMI comms from their PLCs. This is a base function of the PLCs and therefore has been allocated priority for processing with the limited power available.
The Ignition S7 drivers utilise the S7 protocol to get larger and more responsive amounts of tags from the PLCs. The drawback is that Siemens refused to reveal how to browse for tags with this driver and use symbolic names, hence needing to direct address non-optimised blocks for this to work. In 8.3, the new S7 driver is a set of libraries that IA has licensed from a company that has reverse engineered the symbolic S7 protocol, and as a result you can use symbolic access with this more efficient native driver.
The choice of PLC is no longer fully dictated by the controls needed, and now the CPU processing and communications capabilities are critical if you intend to replicate a DCS with your SCADA.
You can read the top level "HSI" tag as a whole and it only counts as 1. But you have to deal with receiving the value as a large Document (JSON). It's easier if you don't need to write to the tags...
