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Maximum Slave number vs Maximum devices Modbus Master

Created by: Frankyphs at: 5/10/2021 4:43 AM (3 Replies)

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5/10/2021 4:43 AM	Rate (0)
Frankyphs Regular Member Joined: 12/22/2020 Last visit: 10/29/2021 Posts: 11 Rating: (1)	Hi guys, now i am having a procurement's power meter project. I am planning to use PLC S7 1200 to handle total 71 power meter Siemens Sentron PAC 3100. I will use Modbus communication module 6ES7241-1CH32-0XB0. While i read the manual data sheet, there is statement like on the picture. My question is, what number of plc should i use to handle 71 slave? I still dont understand the difference of maximum slave number and maximum devices means? below is the modbus communication module i used. thank you :) Attachment WhatsApp Image 2021-05-10 at 09.09.53.jpeg (909 Downloads)

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5/10/2021 6:55 AM	Rate (2)
ALI_N23 Experienced Member Joined: 4/10/2021 Last visit: 3/21/2024 Posts: 70 Rating: (6)	Dear Frankyphs When you use modbus protocol to connect and communicate with other devices there are 2 type of limitation. first one is the modbus protocol limitation about max slaves which can connect to a master. (as you know modbus protocol is a master/slave protocol : master ask the slaves and they answer). this limitation is 247 . the second one is the limitation of your physical layer. for sure there are some physical limits to RS485 or what another port you are using as the physical link to the devices. RS485 limits you to 32 devices, (although you can extend that with repeaters, with some consideration). be aware that modbus rtu is not RS485 (modbus usually communicates through RS485 port of devices). about solution for your project based on my experience with modbus I suggest not to use PLC for modbus communication. I have very good experience with DELTA HMIs (HMIs made by delta electronics company). these HMIs have built in instructions for modbus and what you need to do is just connect devices and HMI to a network (using rs485 port of HMI) , define comm. parameters and them using "NUMERIC DISPLAY" element of HMI screens and specify the address you want to read. It will read the value and show it to you. (no need to write any instruction). About 71 devices of your project I am not sure how many devices you can connect to these HMIs (maybe less or more than 32), but I know some of these HMIs have 2xRS485 port and maybe you can use both. Other benefit is that the HMI prices are reasonable and HMI configuration software free to download from Delta website. I suggest first take a look at manual i addressed (especially page 75 and 100)( http://www.deltronics.ru/images/catalogue/IABG_EN_20181002_web.pdf ) and then contact with your country's local distributor of delta products, they can help you more. [I should say that I have no relation with Delta company and not trying to advertise their products].

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This contribution was helpful to 1 thankful Users Frankyphs

5/10/2021 5:38 PM	Rate (1)
danw Gold Member Joined: 3/28/2010 Last visit: 11/8/2024 Posts: 1074 Rating: (214)	1. > I still dont understand the difference of maximum slave number and maximum devices means? Any given slave can have an address in the range 1 through 247. So for example a small RTU over RS-485 network might have 5 slaves, arbitrarily assigned slave node addresses 1, 7, 33, 105, 241. (not typical, people normally start and one and increment by one as they number the slaves; but the example is valid and it would work) The Maximum number devices refers to the electrical 'load' that any given master or slave RS-485 port driver can drive, which is a maximum of 32 loads, meaning master = 1 load 31 slaves = 31 loads total = 32 loads More then 32 loads and there's not enough electrical oompf to reliably drive more loads and the entire network suffers from bad electrical signals. An isolating repeater repeats whatever it sees on its input at its output and a repeater's output can drive another 31 loads. 2. Issues with single RS-485 network with lots of slave nodes A. Data throughput Only one slave can be polled at a time. The total time to poll 71 slaves on a single could be considerable. How often does data need to be updated? If only every minute, then it's probably doable. B. faulty/missing/disable slave does not reply and the master waits to 'time-out' A fully functional lots-of-slaves network can have an optimal throughput time, but that throughput time is out the window when a slave does not respond, because typically the master's timeout is on the order of one to multiple seconds to allow it to wait for a slave reply and that time out period adds to the optimal throughput period. Any RTU network with lots-of-slaves should have some means of removing a faulty/missing/disabled slave from the master's polling schedule so that the optimal throughput rate is not bogged down when slaves have a problem. C. Daisy Chain wiring An RS-485 network is daisy chained: master to slave 1, slave 1 to slave 2, slave 2 to slave 3 . . . . That makes for a long chain from beginning to end. That all-devices-on-one-network. A loose connection or damage to a cable the creates an open circuit in the middle of the daisy chain could disconnect everything 'downstream' of the open circuit location, creating time-outs for all devices downstream. Sometimes it just makes more sense to use multiple RS-485 networks because of the geographical layout of the site and in so doing, it reduces potential issues with all-devices-on-one-network.

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6/19/2021 4:48 PM	Rate (0)
ola84 Bronze Member Joined: 2/22/2016 Last visit: 10/6/2024 Posts: 141 Rating: (30)	Hi Frankyphs I see that you already have some good answers and suggestions from mihesper400 and danw. I you are going to use a Siemens S7-1200 PLC, I have a library a believe will be very helpful for you when you have multiple modbus devices on a single bus, especially with as many as 71 devices. One of the features is that the library handles disconnected devices, so the PLC would not need to wait unnecessarily long on timeouts. Please check out: https://github.com/rexhip/rexhip, and please ask you need any help. Kind regards, Ola
	Please use the contact form in the link below if you will like to come in contact with me. I don't check personal messages frequently. https://sn7.no/contact/rexhip
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