Reading array of strings in Micrologix returns only first string about libplctag HOT 9 CLOSED

After some experimentation I have some results that are annoying. While reading multiple strings works with a MicroLogix, you can only read a maximum of two strings because of the response packet size limitations. For some DH+ connected SLC models (5/01 and 5/02 apparently), the limit is so small (41 words) that you can only get one string.

I will modify the library in the following ways:

• Add in logic to handle multiple request packets for embedded PCCC calls.
• Add in logic to parse the PLC/5-style data file names so that I can determine when I need to request one thing at a time (strings) and when I do not (INT). Experimentation will be needed to determine what I can read in groups and what I need to read one at a time.
• Refactor some of the DH+ and PCCC embedded handling so that there is not so much duplication of code.

I am not sure how long this will take. I am pretty busy right now. But it is one of my top priorities.

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Marking this as an enhancement. Apparently this is exactly how AB decided to make the protocol respond. This seems highly annoying.

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As the maximum response size is unknown without experimentation, is it feasible to do:

1. Ask for everything
2. Processor will respond with as much it can, eg two strings.
3. Examine response for completeness of data.
4. If only a portion of the data is responded, ask for the rest.
5. Repeat until all data retrieved or error.

A further enhancement on this would be to then store the expected number of elements returned for a particular processor type / data type / tag, and get it right first time the next time a read is requested.

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Possible? Yes. Feasible? Not with the 1.x version of the library. The problem there is that there is nothing to drive the above sequence of steps. There isn't a thread of control for it. I could build a state machine that sequenced when you called plc_tag_status() on a tag, but that is... not ideal.

The version 2.x (which exists, really!) is shaping up to be able to do this. Not trivial by any means, but a lot more feasible than in the 1.x version.

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Version 2.0 does allow for shorter-than expected responses, but only on Logix-class tags so far.

I think the real fix will be to implement multiple-request support in PCCC. It looks feasible, but it probably will not be a 2.0 feature.

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It may be easier to implement real CSP/PCCC/DF1 support first and then base the solution on the target PLC type. Different CSP PLCs prefer different DF1 dialects.

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I am marking this as post 2.0.

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Now that SLC/MicroLogix support is broken out into two parts, it may be possible to look at this again. I will take this up after I get the Debian packaging and some internal clean ups done.

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Closing this in favor of #114. That will be the general issue for this support.

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