Hi Łukasz, At this point I tend towards the need for a tool that allows one to define the schema of a modbus device with all available values and code to convert them into usable stuff (like in java an actual String or Double). So far it seems this needs indeed flags indicating which values can and must be read combined (like the sync block in SunSpec). Then based on this a request for a set of functional values is converted into a set of modbus requests that follow these defined rules.
Rules about an after connection delay and maximum requests per second seem like something that should be at the modbus level (i.e. part of plc4x). Is there an existing project that does something like this? Niels On Sun, Jan 15, 2023 at 5:22 PM Łukasz Dywicki <[email protected]> wrote: > I am really glad you are moving it forward and fighting it! > > It is fairly difficult to get modbus code aligned and working > automatically because different devices behave differently with even > most basic requests. > I remember that one of electric meters I had to work with did support > read multiple registers but did not support going over "empty" indexes. > By empty I mean these which were not declared in the documentation, > even if it was still below 125 limit. > > Effectively I could draw three primary options to cover: > - read continuous (just keep expanding requests up to the limit) > - read blocks (group indexes into blocks up to limit, if they are > continuous) > - read single (read multiple registers is not supported) > > Some devices, especially older PLCs may also require extra delay after > data retrieval, so you have to wait between finishing one request and > starting another. Some inverters and official dongles made by fairly > large Chinese manufacturer, which brand I won't mention here, as well as > primitive tcp/rtu bridges, may require "after connection delay" in order > to establish rtu connection they use to serve tcp session. > > All this makes modbus option list rather long, compared to abilities it > gives! > > Cheers, > Łukasz > > > On 15.01.2023 17:05, Niels Basjes wrote: > > Hi, > > > > No, I'm reading many logical values where the needed registers combined > > exceed the 125 mark. > > Assume my application needs a set of values and asks for such a block of > > registers. > > Then the modbus library that gets the request for 200 registers can no > > longer reliably split it into multiple parts that yield the correct > value. > > > > What I understand of your S7 example (I do not know this device) seems > like > > a device that offers a single value that exceeds the limits of modbus and > > thus I expect they made it so you can fetch it step by step. > > > > I'll see if I can do a wireshark dump for you. > > > > What should I make the issue say? I currently do not see this as a bug or > > problem with plc4x. > > > > Niels > > > > On Sun, Jan 15, 2023 at 4:46 PM Christofer Dutz < > [email protected]> > > wrote: > > > >> Hi Niels, > >> > >> I think this might be related to what happens when reading Strings in S7 > >> without providing a length. > >> In this case each string is 255 charaters long and it already exceeds > the > >> size-limit of 200 or so of a S7 1200. > >> > >> In S7 we use some code to split up multiple tags into multiple requests, > >> but rewriting the query to support automatic splitting of one single tag > >> into multiple requests is quite a bit more difficult. > >> > >> I thought you were reading multiple tags that in sum exceeded the 125 > >> registers and for that case simply something similar to the S7 query > >> updater should also work for other protocols. > >> > >> Do I understand you correctly, that in your case you want to read one > tag, > >> that is bigger? > >> > >> Chris > >> > >> From: Niels Basjes <[email protected]> > >> Date: Sunday, 15. January 2023 at 16:38 > >> To: [email protected] <[email protected]> > >> Subject: [Modbus] Handling 'too large' blocks of registers > >> Hi, > >> > >> A few weeks ago some code of mine was merged to limit the number of > >> registers that can be requested in a single ModbusTag because asking for > >> more than 125 registers in a single request will always fail (because of > >> the way modbus works). > >> https://github.com/apache/plc4x/pull/721 > >> > >> One of the comments was asking if it can be automated in a generic way > to > >> split a "too large" request into multiple smaller requests. > >> So If I ask for a block of 200 registers then plc4x would simply split > it > >> into multiple requests (like 125 and 75 for example, or 100 and 100) and > >> afterwards merge the resulting registers of the two requests back into a > >> single block of 200 registers for the upstream application to analyze. > >> > >> I said I would look into this and this is what I found while doing some > >> experimenting with my real solar converter. > >> This device uses the SunSpec standard for defining meaning to modbus > >> registers. > >> > >> My current conclusion at this point is that it is impossible to handle > this > >> at the modbus level and I'm looking for you guys to challenge me in > this. > >> > >> Why do I say this? > >> One of the logical values you can retrieve is the name of the > Manufacturer > >> and the Model of the device that are both stored as UTF-8 in a set of 16 > >> registers (i.e. usually 32 characters). > >> https://github.com/sunspec/models/blob/master/json/model_1.json#L27 > >> https://github.com/sunspec/models/blob/master/json/model_1.json#L36 > >> > >> I found that if I try to fetch only a part of this logical value I get > >> either an error (INVALID_ADDRESS) or bad data. > >> When I fetch the entire value I get the 16 registers which hold the > correct > >> data. > >> > >> Output of my test program that fetches the Model of the device (which > is in > >> my case "SB3.6-1AV-41" ). > >> ModbusTag[4x40021:UINT[16]]: 0x5342 0x332E 0x362D 0x3141 0x562D 0x3431 > >> 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 > >> ModbusTag[4x40021:UINT[15]]: INVALID_ADDRESS > >> ModbusTag[4x40022:UINT[15]]: 0xFFFF 0xFFFF 0xFFFF 0xFFFF 0xFFFF 0xFFFF > >> 0xFFFF 0xFFFF 0xFFFF 0xFFFF 0xFFFF 0xFFFF 0xFFFF 0xFFFF 0xFFFF > >> > >> So if I retrieve the correct block I get it. > >> If I try to get 1 register less than I should I get an error. > >> If I try to get the registers starting from the 2nd register I get only > >> 0xFFFF (important: NO error and a PlcResponseCode.OK !!) > >> > >> I expect there to be many other modbus devices that have similar effects > >> when the logical value requires multiple registers. > >> > >> So for now I come to the conclusion that fetching only part of the > >> registers that make up a logical variable in general will not work and > (in > >> some cases) cannot be detected to have failed. > >> > >> At this point my assessment is that the only way to handle this is by > >> actually knowing the meaning of the block of registers (i.e. which must > be > >> read together in a single read) and work from there. > >> > >> Note that the current SunSpec definition also has the concept of a > 'sync' > >> group of points (logical values that consist of 1 or more registers) > which > >> are grouped in the specification with the intent of "indicating > >> that the points in the group must be read and written atomically", which > >> effectively means you cannot read them individually from the modbus > >> standpoint. > >> > >> Looking for your feedback on this. > >> Do you know of existing software to generically handle this ? > >> > >> -- > >> Best regards / Met vriendelijke groeten, > >> > >> Niels Basjes > >> > > > > > -- Best regards / Met vriendelijke groeten, Niels Basjes
