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Initial value for first S7-200 run

Created by: K-Dawg at: 9/11/2007 2:08 AM (14 Replies)

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9/11/2007 2:08 AM	Rate (0)
K-Dawg Posts: 20 Rating: (0)	Hi all, In spite of all my efforts, I can neither figure this out nor find a solution anywhere. I would like the PLC to holda default value in V or M memory but only during the first time a program is ran. This means, that the value should be available only after first downloading the program. After that, I would like to modify the contents of memory as the program is restarted everyday. That way, the user can have a default (but modifiable) value when he first uses the PLC.
K-Dawg Posts: 20 Rating: (0)	Last edited by: K-Dawg at: 11.09.2007 02:08
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9/11/2007 4:09 AM	Rate (1)
rosa Posts: 191 Rating: (58)	Hi!! i´m not sure if i understand what you need.. try this: > on Step7 Microwin open the data block icon > type the initial values you want MW0 10 // this loads a 10 in the MW0 memory VD100 15.8 // this loads a 15.8 in the VD100 memory > download the program at this point the memory areas will have the values you programmed ifyou changea value it will accept the new value and keep it ( you can stop, start, turn off, etc the PLC ) if you want to apply the original initial values you only have to download again the data block to the PLC.. ¿¿ is this what you are looking for ?? Hope this helps!! ROSA
rosa Posts: 191 Rating: (58)
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9/11/2007 5:03 AM	Rate (0)
K-Dawg Posts: 20 Rating: (0)	Rosa, Thanks for your help. I will try out your suggestion first thing in the morning. I think it will work.
K-Dawg Posts: 20 Rating: (0)
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9/11/2007 4:43 PM	Rate (1)
rosa Posts: 191 Rating: (58)	Ok!! please post again if this is not what you are looking for.. feedback helps! please always rate the answers you receive. ROSA
rosa Posts: 191 Rating: (58)
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9/11/2007 7:36 PM	Rate (0)
K-Dawg Posts: 20 Rating: (0)	Rosa, I tried out your suggestion and it worked. Unfortunately, data block memory assignments are restricted to just V memory. But I should find a workaround. Thanks.
K-Dawg Posts: 20 Rating: (0)
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9/11/2007 9:04 PM	Rate (1)
Mojo Bronze Member Joined: 3/27/2006 Last visit: 10/11/2024 Posts: 90 Rating: (13)	M memory is volatile because it cannot be part of the Data Block. Stick with V memory if you need it to be retentive or you want to be able to suck it back for backup on your laptop. On jobs in which it is not cost-effective to use a battery backup I wrote code that detects a change in setpoint. When a change is detectedthe programwrites the new setpoint to EEPROM memory. This means it updates the onboard storage of the data block you loaded previously. This eliminates all concern for "safe" setpoints similar to the data block method while providing the next step: instantrestoration of the running values. An example program is attached. Attachment EEPROM.zip (440 Downloads)
	"I never let a cowboy make the coffee." Yeah, thats what Granny always said to my Grandad. And he'd say: "Never tell a joke that ain't that funny more than once." "And if you wanna hear God laugh, tell Him your plans." Van Zant "Help Somebody"
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9/11/2007 11:15 PM	Rate (0)
K-Dawg Posts: 20 Rating: (0)	Mojo, Thanks a lot for your useful input. Instead of writing V memory to EEPROM, I have been declaring the first 13 bytes of M memory as a retentive range thus automatically saving them to permanent memory on power down. I see no problem with this implementation, especially because I don't have to write code for saving V memory to EEPROM (which is what I was going to do initially). Could you please relay to me any drawbacks you may see with this? Aside from that, I may have to manually write V memory to EEPROM as opposed to using to retentive M memory, depending on how I'm going to use the data block.
K-Dawg Posts: 20 Rating: (0)
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9/12/2007 3:44 PM	Rate (1)
Mojo Bronze Member Joined: 3/27/2006 Last visit: 10/11/2024 Posts: 90 Rating: (13)	When you don't use a battery cartridge the S7-200 has capacitor-backed memory. Siemens says its good for 72 hours. Keep this in mind. When you load a plc program to the cpu you have the option to load a data block. NO DATA BLOCK LOADED: If you don't load a data block then the plc starts out with everything at zero. If you have setpoints to enter via a touchscreen these setpoints are in RAM. They are not permanent because they are not in EEPROM. They are backed up by the battery cartridge or if no battery is installed - the capacitor. In the event the power is off long enough you will lose these setpoints in RAM. With no data block loaded your cpu will power back up with everything at zero. You can see how this would cause a problem with machine motion! DATA BLOCK LOADED:At initial startup the plc will use the values from the data block - this is a one-time deal. Then you could use a touchscreen or your laptop to modify these values. Once again these values are in RAM. They are not permanent because they are not in EEPROM. They are backed up by the battery cartridge or if no battery is installed - the capacitor. THE DIFFERENCE is if the power is off long enough for the RAM to go bye-bye, at power up the plc will use the values from the data block. When your program writes to the EEPROM it is writing to the data block. This solution gives the best of both worlds: you get to keep the current running values if the power is off long enough for the capacitors to lose their charge (or the battery cartridge fails). Because M memory cannot be stored in the data block it is not possible for this solution to work with M memory. If I were you I'd switch to V memory or use a battery cartridge.
	"I never let a cowboy make the coffee." Yeah, thats what Granny always said to my Grandad. And he'd say: "Never tell a joke that ain't that funny more than once." "And if you wanna hear God laugh, tell Him your plans." Van Zant "Help Somebody"
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9/12/2007 4:02 PM	Rate (0)
Mojo Bronze Member Joined: 3/27/2006 Last visit: 10/11/2024 Posts: 90 Rating: (13)	On topic: After you get your process running the way it needs to run (tuned), you can set the plc to stop mode and execute the command: Create Data Block From RAM. This will write all of the RAM values to EEPROM and give you the option toupload them to your laptop. Once again - doesn't work with M memory.
	"I never let a cowboy make the coffee." Yeah, thats what Granny always said to my Grandad. And he'd say: "Never tell a joke that ain't that funny more than once." "And if you wanna hear God laugh, tell Him your plans." Van Zant "Help Somebody"
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9/12/2007 6:45 PM	Rate (0)
K-Dawg Posts: 20 Rating: (0)	Mojo Jojo, I have a variable whose value won't change too often. I think I will use V memory for this and write to EEPROM whenever a change is detected, much like your sample program suggests. However, I think it's still a good idea to use M memory for some other variables. These involve an hour meter, whose value is changing continuously. We certainly do not want to write to EEPROM everytime a second or a minute elapses. I think it would be suitable to use the M memory special feature that writes M memory data to EEPROM upon power down. I do this because it's a better alternative to saving all of the RAM in EEPROM. Or can you save just part of the RAM in EEPROM on power down apart from using MB0 to MB13?
K-Dawg Posts: 20 Rating: (0)	Last edited by: K-Dawg at: 12.09.2007 22:54
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