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Web server write DB referenced by HMI or FB

Created by: sulliwane at: 8/1/2014 10:54 AM (3 Replies)

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8/1/2014 10:54 AM	Rate (0)
sulliwane Posts: 5 Rating: (0)	Hi all, I'm newbie to automation, and particularly interested into the web-server capacities of the S7-1200. I already played with AWP commands and succeed to READ and WRITE a DB_tag using Jquery (thanks to the examples in the forum). But as soon as the DB_tag is referenced by a FB or by a screen in the HMI, I find it impossible to WRITE it from the browser (but still possible to READ). Can you confirm that ? Is that normal from an automation point of view ? (variable reserved by FB or HMI for writing). Thank you for you input !
sulliwane Posts: 5 Rating: (0)
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8/1/2014 11:38 AM	Rate (0)
speder Posts: 3 Rating: (0)	Hi! You have to check that, wheter your FB overwrite the DB_tag, or the HMI. In this case the data you send from the webserver to CPU immediately will be overwritten by the FB or HMI.
speder Posts: 3 Rating: (0)
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8/1/2014 1:32 PM	Rate (1)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 8/29/2025 Posts: 16048 Rating: (2512)	If you want to have multiple sources directing your plc logic, you may want to consider having unique tags for each "command source". Like an HMI specific DB that handles commands from the HMI, WWW specific DB that handles commands from the web server. Then a command interpreter that figures out what to do when the two commands are in conflict. For example, if your last HMI command is "ON", and then some time later, the WWW command comes in as "OFF", your interpreter may do something like this (pseudo code): If (HMIcmd="ON" more than 5 sec) and p-trig(WWWcmd="OFF") then HMIcmd = "OFF" Turn OFF device/signal ELSE IF (HMIcmd="OFF" more than 5 sec) and p-trig (WWWcmd="ON") then HMIcmd = "ON" Turn ON device/signal END IF In this way, the HMI always has control except for the rare www signal state change, and only if the hmi has not been touched recently. There are many ways to get the desired result. This is only one such way to get this specific result. It may help you think about your www command handling differently and help ease troubleshooting for the future.
	science guy
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This contribution was helpful to 1 thankful Users sulliwane

8/2/2014 8:01 AM	Rate (0)
sulliwane Posts: 5 Rating: (0)	Ok, thanks both of you for your answers. @huggy_d1 : I got your meaning. I will try to put this into practice next week.
sulliwane Posts: 5 Rating: (0)
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