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Modbus server

Created by: dnordenberg at: 5/13/2017 12:19 AM (6 Replies)

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5/13/2017 12:19 AM	Rate (0)
dnordenberg Posts: 5 Rating: (0)	Hi! I don't have much luck with using modbus server functionality. I tried a network project. First attaching a server in first connector, client on the second connector. I can not pull any data from the first connector that is on port 502. I then add a server on the third connector which get port 504 (non standard modbus port). Here I get data on holding register 0. Why doesn't port 502 work then? I also laborated a bit with the data transfer table in server mode but it seems that it doesn't do anything there, it's what logo settings/modbus address space sais that counts. Why is the data transfer table even there in server mode then? Third strange thing I've noticed, data from the client is polled to VW0, I then fetch that with a network analog input block and store it to a network analog output block on VW1. I then watch both VW0 and VW1 with then data table, they are the same but then I poll then on modbus and they are not the same there! Very similar value but VW1 is slightly offsetted. See attached picture. Attachment Skärmklipp.JPG (327 Downloads)
dnordenberg Posts: 5 Rating: (0)
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5/13/2017 12:23 AM	Rate (0)
dnordenberg Posts: 5 Rating: (0)	project attached... Attachment networktest.zip (280 Downloads)
dnordenberg Posts: 5 Rating: (0)
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5/13/2017 5:57 AM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 1/1/2026 Posts: 36393 Rating: (4545)	What exactly is wrong with your connections, I can not say. I would connect to a MODBUS compatible device, as it is in the attachment. When using VW0 and VW1, the problem arises that the words overlap. VW0 includes the bytes VB0 and VB1, VW1 includes bytes VB1 and VB2. Both words thus use byte VB1 and influence each other. Therefore your different values. By the way.... You do not need a HIGH block for the message text. In order to activate the message text, it suffices to negate the input connector. In the attachment is a program file with my suggestion and a video, where you can see how the overlap affects. Attachment network_V2.zip (300 Downloads)

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6/30/2017 3:57 PM	Rate (0)
dnordenberg Posts: 5 Rating: (0)	Hi! Sorry for the delay, I simply can not get this to work on port 502 :( Is there a firmware bug maybe? Has someone else gotten it to respond on standard modbus port 502 in server mode? Can this be affecting only this single logo unit? Thanks for the tip on VW registers, I did try to find out more about them in the help, how they worked and are addressed but I could not find this info :( Now I know I can only use every other registrer :) I'm more used to this happening with 32 bit registers on systems using 16 bit ad default.
dnordenberg Posts: 5 Rating: (0)
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6/30/2017 4:33 PM	Rate (0)
dnordenberg Posts: 5 Rating: (0)	Started over on a new project, first in diagram mode where you can only have one server and it worked! Then i switched to network mode, added a server on the first port and that worked too! I guess something went wrong with my first project, something that could not be fixed by adding, removing connections :( Anyway now it works :)
dnordenberg Posts: 5 Rating: (0)
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This contribution was helpful to 1 thankful Users ixo65

6/30/2017 4:50 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 1/1/2026 Posts: 36393 Rating: (4545)	dnordenberg Started over on a new project, first in diagram mode where you can only have one server and it worked! Then i switched to network mode, added a server on the first port and that worked too! I guess something went wrong with my first project, something that could not be fixed by adding, removing connections :( Anyway now it works :) Thanks for the feedback. Maybe it can help other users with similar Problems.

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This contribution was helpful to 1 thankful Users dnordenberg

6/30/2017 5:06 PM	Rate (0)
dnordenberg Posts: 5 Rating: (0)	Another thing to keep in mind, when adding a modbus device in network mode and drag a connection from this device to a free port on the logo, you need to be careful with the IP of that device then as the logo seems to lock to request only from this IP then. If you on the other hand right click on a port and just add server connection it seems like it is then open for requests from all IPs. Guess this is like a security thing making it possible to lock down the IP which it will accept requests from. Also the direction you drag the connection is important, device to logo making the logo a server, logo to device making the logo a client. This seems to be not documented in the help :(
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