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Modbus RTU a few slaves

Created by: mal_er at: 12/11/2014 10:37 PM (6 Replies)

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12/11/2014 10:37 PM	Rate (0)
mal_er Posts: 32 Rating: (1)	As the topic question modbus rtu and several slaves. How to create the program if I have 10 slaves? - One MB Master (MB Addr 0 and polling of all of the same) - Or 10 slaves and 10 MB Master (any other MB Addr and any registers). If the no. 2 is that each MB Master has the same or different DB. If different, how to set the parameter in MB_COMM_LOAD MB_DB?
mal_er Posts: 32 Rating: (1)
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12/12/2014 10:22 AM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	You can only have 1 master on the network and up to 31 slaves (RS485). The master does not have a slave address - only slaves have slave addresses. So you would give each slave a unique slave address - this is very important. If any slaves have the same slave address chaos will ensue. As modbus is a hlf-duplex protocol, you can only have one master-slave transaction in progress at a time. So you will have to write your modbus code so that it issues a request to slave number 1. It then waits for a valid response or a timeout. If you get a valid response then you can store that data and then move onto issuing a request to slave number 2, then repeat for all slaves. If you get a timeout, then you will have to decide what action to perform. You could:- a) keep trying that slave until it finally responds. However, if the slave never responds for some reason, then you will never move onto the other slaves and the link will effectively be stalled. b) Give up and move onto the next slave immediately on timeout c) A amalgamation of a) and b). Give each slave a number of retries to sucessfully respond (say 3). If after 3 retries the slave fails to respond, then you move onto the next slave. This is what I have done in the past and it works well. The retry counter would be reset to zero before issuing the request to the slave so that each slave always gets 3 retrires.
	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...
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12/12/2014 4:19 PM	Rate (0)
mal_er Posts: 32 Rating: (1)	So for a few slaves, there is only one block MB Master. I just need to swap MB addr for the next slaves. Is that right?
mal_er Posts: 32 Rating: (1)
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12/13/2014 11:14 PM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	Yes in esence that is it. Wait until you get DONE or ERROR. If DONE, disable the send block, change the configuration so that it is accessing the next slave (e.g. change slave address to be polled, DB etc.). Then re-enable the send block. If ERROR then keep the setup as is until the retries have been exceeded then give up and proceed to the next slave as you did if DONE were set.
	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...
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This contribution was helpful to 1 thankful Users mal_er

12/14/2014 12:48 AM	Rate (0)
mal_er Posts: 32 Rating: (1)	Thank you very much for your help. The program I have ready and I did exactly as you wrote.
mal_er Posts: 32 Rating: (1)
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12/15/2014 10:37 AM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	Did it work?
	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...
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5/13/2015 5:15 PM	Rate (0)
Min_Moderator Platinum Member Joined: 9/3/2014 Last visit: 3/13/2019 Posts: 4766 Rating: (122)	New question published by pedroso is split to a separate thread with the subject Send two values at the same time with modbus rtu. Best regards Min_Moderator

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