Skip to Content

Register Login

Industry Online Support

Technical Forum

Navigation

Modbus Rtu slaves base address ?

Created by: Grendizer at: 2/21/2024 8:16 PM (5 Replies)

Rating (1)

Thanks 1

6 Entries

2/21/2024 8:16 PM	Rate (0)
Grendizer Joined: 2/12/2024 Last visit: 7/1/2024 Posts: 14 Rating: (0)	Kindly, we are trying to do some Modbus Rtu communication between a 1214C plc with CM1241 module, and the following slaves. (Schneider PM5110 power meter) (Socomec Delphys GP ups) We need to read some analog variables from these slaves, using the plc usual ladder logic blocks. We got the register lists for each slave, but we are not sure if we should add to the address the value 30001 as mentioned in some Modbus Rtu sites. Is it always required for S7-1200 Modbus Rtu communications? Thank you
	Last edited by: The Helping Hand at: 02/22/2024 13:08:52 Text formatting optimized
Suggestion To thank Quote Answer

2/22/2024 9:10 AM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 10/21/2024 Posts: 1547 Rating: (537)	This depends on the how the driver was designed. Modbus drivers have 2 addressing modes: Address (application layer) or offset (protocol layer). Some drivers require the address mode and some the offset mode. You would have to read the documentation for the driver block that you are using. Or you could try both and see which works. It will do no harm.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
Suggestion To thank Quote Answer

2/22/2024 7:23 PM	Rate (0)
Grendizer Joined: 2/12/2024 Last visit: 7/1/2024 Posts: 14 Rating: (0)	You mean that it depends on the brand ? For example, if i want to read the Active Power Total (register 3060) , i should do MB_MASTER with DATA_ADDR (3060) (+-1) ? Or, as per below links, do MB_MASTER with DATA_ADDR (3060 + 30001) (+-1) ? Modbus RTU made simple with detailed descriptions and examples (ipc2u.com)

Suggestion To thank Quote Answer

2/25/2024 10:46 PM	Rate (1)
danw Gold Expert Joined: 3/28/2010 Last visit: 10/31/2024 Posts: 1074 Rating: (214)	Yes. Everything depends on brand, model, software and implementation of the firmware/software. Hexadecimal addressing is almost always zero based, so the first value is at address 0x0000:. 0x0000 is the first register in a 0-based map, 0x0001, 0x0002 0x0001 is the second register in a 0-based map 0x0002 is the third register in a 0-based map Smiffy calls this protocol based addressing because the actual address in the Modbus PDU message is zero-based, starting at zero, as shown above Decimal addressing is usually one-based, and is based on the Modbus "Memory Area", where Input Registers are identified with the leading numeral (3), as in (3)xxxx or (3)xxxxx and Holding Registers are identified with the leading numeral (4), as in (4)xxxx or (4)xxxxx, such as (4)3060 or (4)03060. Those register maps using the Memory Area typically start at number 1 for the first register, hence the term one-based: (4)00001 is the first register in the Holding register map (4)00002 is the 2nd register in the Holding register map (4)00003 is the 3rd register in the Holding register map The leading numeral is NOT used in the actual Modbus message, instead the indexed value minus one is the number used in the Modbus PDU message. All that said, the practical method is to read a register with a known value. If you get the wrong value, read the register on either side (plus 1 or minus 1) until you get the correct known value. Then you know that for that master and that slave, that you know what addressing offset is. I use Modscan64 as my generic Modbus master. It uses one based addressing. When I talk to Modbus slaves I've used before, I know from my notes what the addressing offset is. When I talk a new device, I poke around for a known value to read to confirm the addressing. The PM5110 memory map here: https://www.se.com/us/en/faqs/FA234017/ says it starts at 1, although it has no variable listed for register 1. (3)03060 Active Power Total is a 32 bit float, so you need to read 2 registers. There are 4 IEE4754 word/byte order formats for 32 bit floating point values, so you are also faced with a getting the right word/byte order in addition to getting the correct pair of 16 bit registers, 3060 and 3061. Why not read register (3)01837, the year, which presumably is setup correctly and it a single register 16 bit unsigned value, which will not be the month (3)91838 or the Cause of the Last Meter Reset ()3)01836. Getting the right register from the year tell you the offset you need or don't need.

Suggestion To thank Quote Answer
This contribution was helpful to 1 thankful Users smiffy

2/26/2024 5:24 PM	Rate (0)
Grendizer Joined: 2/12/2024 Last visit: 7/1/2024 Posts: 14 Rating: (0)	Kindly, based on your experiences, i want to ask about this issue. In the PM5110 memory map, there are some gaps, like with the below registers. So, when we read a big range of variables, will we get the correct data? Will it get through the gap? Displacement Power Factor Total 3092 --- 2 4Q_FP_PF R Frequency 3110 Hz 2 FLOAT32 R Power Factor Total IEC 3192 --- 2 FLOAT32 R https://www.se.com/us/en/faqs/FA234017/

Suggestion To thank Quote Answer

3/14/2024 4:29 AM	Rate (0)
danw Gold Expert Joined: 3/28/2010 Last visit: 10/31/2024 Posts: 1074 Rating: (214)	@ Grendizer >So, when we read a big range of variables, will we get the correct data? Will it get through the gap? There is NO way to answer this question until you actually try reading a series of registers to see if you get a valid response or not. That's because there are hundreds of thousands of Modbus devices and all of their implementations reflect the relative lack of concern by the vendor for documenting whether or not unspecified registers can be read without generating an exception code. Maybe one in thousand vendors documents that fact, but most do not, so you don't know until you try it. So, you try it and see and read a bunch of registers. If the reply is an exception code 02, Illegal Data Address, then you know that you can't read those registers because the server/slave will throw an error code. That's one reason for using a Windows generic Modbus client/master is to test out this situation because it's generally a lot easier and quicker to change a parameter or two in the Windows app, than doing so in development software.

Suggestion To thank Quote Answer

|

Share this page:

Share this page on...

Follow us on