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SCL S7_1200: Getting the DB number from symbol

Created by: salrandazzo at: 10/29/2013 10:08 AM (5 Replies)

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10/29/2013 10:08 AM	Rate (0)
salrandazzo Posts: 57 Rating: (8)	Hi, I'm trying to use PEEK & POKE instructions on a S7-1200 cpu using SCL Language. I would like to access a DB using symbolic addressing. Booth PEEK and POKE instruction require area = 16#84, then a DBNUMBER (Dint) and a BYTEOFFSET (Dint). Easy if I use absolute addressing (e.g. PEEK(AREA := 16#84, DBNUMBER := 10, DBOFFSET := 120) But what if I would prefere to use symbolic addressing? What if I would peek something like DB_HMI.ALARM[10] ? How to extract DBNUMBER and DBOFFSET? I guess that my DB should be created in 300/400 mode, in order to be able to absolute address it. I could, of course, use absolute addressing. But, as usually happens, if I modify my DB structure, I have to modify every absoulte reference I used in my program. Symbolic addressing helps to automatically resync my program after every DB structure change. THanks for your attention. Salvo
salrandazzo Posts: 57 Rating: (8)
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10/29/2013 2:50 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 11/8/2024 Posts: 15388 Rating: (2443)	For the s7-1200 plc's, you are absolutely right in saying to use peek/poke, you need to have your DB in compatibilty mode (s7-300/400). If you ask a different question, we might have a better answer. What problem are you trying to solve by using the peek/poke? It is possible to solve most problems without using peek/poke. However, I had to resort to iterating through a DB using peek to find the DB size (until I received an error), so I know there are uses for peek.
	science guy
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10/29/2013 3:26 PM	Rate (0)
salrandazzo Posts: 57 Rating: (8)	huggy_d1 If you ask a different question, we might have a better answer. What problem are you trying to solve by using the peek/poke? It is possible to solve most problems without using peek/poke. However, I had to resort to iterating through a DB using peek to find the DB size (until I received an error), so I know there are uses for peek. So... I have an alarm structure based on two DBs. In DB_ALARMS (bit structured) I call alarms by name, and I set/reset them individually. then I copy them in DB_HMI, that is word structured, and works as interface to the HMI panel. When I copy alarms from DB_ALARMS to DB_HMI I have to PEEK bytes from DB_ALARMS and POKE byte to DB_HMI. Why I don't PEEK/POKE by words? Because in the meanwhile I swap HIGH/LOW bytes, as required by HMI. Attached you'll find the HMI alarm SCL module. Sorry about italian Language, I have no time to translate. But SCL code is international. BTW, I have to PEEK and POKE using absolute DB addressing. What a pity that I can't use symbolic addressing. Recently I changed DB number (DB_HMI was DB16 and now it's DB171), and I forgot to update the PEEK/POKE addressing. Imagine the result.... Attachment HMI_1.zip (432 Downloads)
salrandazzo Posts: 57 Rating: (8)
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10/29/2013 3:45 PM	Rate (1)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 11/8/2024 Posts: 15388 Rating: (2443)	It is unfortunate your HMI is not tag based like the basic panels that connect to the s7-1200 PLCs and simply read the tags symbolically. That would save you the DB copy and byte swapping. You might be able to simplify your SCL if you use word slicing. For whatever word you are using, you can directly access the byte like this: let my DB names be PLC_DB and HMI_DB, and inside PLC has an array of INT and an array of WORD in the HMI named ALARMS from 0..10 PLC_DB.ALARMS[1].b0 = low byte HMI_DB.ALARMS[1].b1 = high byte You can set them equal without peeking and poking. There should be a way to accomplish the byte swapping without any peek/poke which in the long term will help make it easier to maintain - as you already noticed when you change the DB #, things are not working as expected (though, you could pass in the DB numbers to your SCL FB and only have to remember to update in that one place.
	science guy
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10/29/2013 4:39 PM	Rate (0)
salrandazzo Posts: 57 Rating: (8)	huggy_d1 It is unfortunate your HMI is not tag based like the basic panels that connect to the s7-1200 PLCs and simply read the tags symbolically. That would save you the DB copy and byte swapping. Strange to say, but I'm using a KTP400. So why do I use this strange alarm procedure? Because I can't succeed to link a bit structured alarm into my HMI. If I declare single bits into the alarm DB (in order to be able to set/reset them individually calling them by name), I can't insert them into the HMI alarm list. When I try to define an alarm trigger (or acknowledge)bit searching it into the PLC DB_HMI, I can't find and use my bit-defined alarms. It only works if they're defined as word array. But this way I would lose the ability to set/reset them individually by name, I should address them as "alarm_word[x] bit y" and so on, loosing the ability to use meaningful names. There is a big difference in calling an alarm as "Water_pump_alarm" instead of "alarm_word[x] bit y"...... Of course, even with my strange procedure I can't succeed having an automatic sync between PLC alarms and HMI alarms, because if I modify the PLC alarm list, I have to carefully resync the HMI alarm list.... The problem lies in the HMI behaviour that doesn't accept to link (as far as I understand) to single booleans defined in a DB. It only likes word array members. Am I right? I'm sure that I still have so much to learn about 1200 using.....
salrandazzo Posts: 57 Rating: (8)
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8/19/2019 10:05 AM	Rate (0)
AutoGen_8522319 Member Joined: 6/18/2019 Last visit: 12/11/2020 Posts: 1 Rating: (0)	Good day, I had the same problem, my approach was to create a function "Bit_poke" that takes a word, an offset and a value as inputs so you can pass the word and set the offset bit to the desired value. Hope it helps you

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