Hi Stephen, Well, it sort of touches that area ... generally big endian/little endian handles the byte order ... so this would more have an effect on the order of the bytes inside the register too. In general I would love to handle BE/LE as an option on the connection string. So per default it's Big Endian (As the spec says), but you could switch to LE, if you need to.
Chris -----Original Message----- From: Stephen Snow <[email protected]> Sent: Freitag, 22. April 2022 12:57 To: [email protected] Subject: Re: [DISCUSS] how to read bool and bool-arrays? Hi Chris and Myhongk, et al Depending on the processor (this would include embedded IIOT devices) they could be big or little endian, which may complicate matters for the modbus driver. Being able to set this as a parameter when using the driver in an app being built which may talk to many modbus devices of either endian arrangement would likely be beneficial. Byte swapping in PLC land is common in communication between PLC's/PC's and periphery devices or other control elements. I don't know if that is what you're referring to, but it sure sounds like endian issues I have encountered in the past (and still today). Be well, Stephen On Fri, 2022-04-22 at 10:26 +0000, Christofer Dutz wrote: > Hi all, > > Myhongk just brought up an issue in the Modbus driver, which I think > could probably also come up in other protocols. > > When reading a BOOL or BOOL[] the driver generally reads chunks of 16 > bits (words). Now we need to define how we count the bits. > > Currently when reading a simple BOOL we do this (0 = skipped, 1 = > read): > > 00000000 00000001 > > So, the least significant bit is the value and the ones before are > skipped. > > When reading an array, we currently start at the end. So reading > BOOL[6] would be this: > > 11111100 00000000 > > In this case we start at the most significant bits. > > Both variants have its logic and it's downsides. > > Downside by starting at the least significant bits is as soon as we > read more than 16 bits. > So, if we read BOOL[19] we'd probably have this > > XXXXXXXX XXXXXXXX 00000000 00000XXX > > Where the indexes in the array would probably be: > > 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 X X X X X X X X X X X X > X 18 17 16 > > Or would it be different? > > Another option would be to start at the end of the last word read and > to count "from right to left" > > > If we start at the most significant bits, it would make things easier > for multiple elements, as we'd have this for BOOL[19]: > > 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 X X X X X X X X > X X X X X > > But we would have only read the register value: 32768 or 0x8000 as > "True". > > > Would be cool to know what's the default industry way to do thins (I'm > sort of expecting an "it depends"). > Possibly we could make it configurable (Which would complicate things > a bit). > > Another thing is ... in S7 we can address parts of a byte as BOOL by > using a Bit address ... would it be cool to extend the Modbus to allow > this too? > > So something like this would work: > > 500001.4:BOOL[3] > > Which would return (assuming we start counting at the most significant > bit): > > X X X X X 1 2 3 X X X X X X X X > > > Feedback highly appreciated :-) > > Chris
