help needed regarding an energy monitoring project

[ptoke01 0]

Hi All We have a project wherein we have 9 energy meters capable on communicating with Modbus RTU ( Rs-485) and we need to take Instantaneous values of both Active power (P) and reactive power (Q) from each meter into a PLC and then add the active power for all the 9 meters and store it in a register say X and add the reactive power of all the 9 meters and store it in a register say Y and then pass on these values of X &Y on to a third party data logger on Modbus TCP/IP. I need to do all this under 100 ms.The max distance between the meters is 200 mts and the PLC i plan to use is Schneider and meters we have PAC 3100 with RS-485 modbus.My query is that for getting the required data under 100 ms how can I calculate the network speed? Is there a formula? Thanks prasad

[Steve Bailey 112]

You won't be able to get data from nine meters in 100 milliseconds over an RS485 cable. It would be a challenge to get the data from a single meter at that rate. You would need to use Modbus TCP and have a PLC capable of maintaining nine simultaneous channels to do it.

[bits'bytes 0]

Hello, see attached pdf for some theoretical timing values. I was assuming a request takes maybe 6 Byte, an answer maybe 10 Byte. So, depending on the baudrate you can configure you have 'pure' traffic time to transfer the requests and answers. I think you may have a chance to achieve the goal when you can design a structure where you query all 9 devices at once. than you will receive the answers one by one. so: not query one, wait for answer, query next, wait for answer, .... but: query, query, query, ..... query last, wait for answers. I can well imagine it works. BR BB Mappe1.pdf

[Steve Bailey 112]

The Modbus RTU protocol doesn't support multiple queries simultaneously. When the master sends a request for data from the slave it has to wait for the response or the timeout period before issuing another request. The strategy bits'bytes outlined can't be implemented in Modbus RTU over an RS485 cable. It can be done with Modbus TCP over an Ethernet cable.

[bits'bytes 0]

Hi Maverick, thank you for clarifying this situation. BR BB

[mohssen 0]

You can use Ethernet (Modbus TCP) to have faster communication and all devices can be accessed at the same time. In this case you need RS-485 to Ethernet Gateway for each power meter. You may use tGW-712 or tGW-715 or similar products: http://www.icpdas-usa.com/tgw_712.html?r=mohsen http://www.icpdas-usa.com/tgw_715.html?r=mohsen If you want to stay with Modbus RTU and using RS-485, You can use RS-485 Hub. you can use it in 2 or 3 level to cover all of your slave devices. It means you connect all of your devices (slaves) to RS-485 hub and there will be one RS-485 out(master) to be connected to PLC. Regarding the delay time, you can check it out in its manual or contact the manufacturer via website. You may use below products: I-7514U: http://www.icpdas-usa.com/i_7514u.html?r=mohsen I-7513: http://www.icpdas-usa.com/i_7513.html?r=mohsen

[a062549 5]

What brand of Power Monitors are you using??