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Am I right in thinking that whenever I want to send a decimal value to the output word I must first use CAW to swap the bytes around?

Created by: RandomCoder at: 10/19/2010 1:53 PM (2 Replies)

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10/19/2010 1:53 PM	Rate (0)
RandomCoder Experienced Member Joined: 8/16/2010 Last visit: 1/19/2022 Posts: 50 Rating: (8)	I was attempting to address an output word using a decimal value. Please could someone tell me if I'm approaching my problem correctly... I have 12 outputs connected to the plant, addressed as shown below. Addr -Symbol - Comment Q6.0 - Setpoint_Bit_00 - Bit 0 of 12 bit code Q6.1 - Setpoint_Bit_01 - Bit 1 of 12 bit code : : : : :: Q7.3 - Setpoint_Bit_12 - Bit 12 of 12 bit code The remaining bitsof output Q7 (i.e Q7.4 - Q7.7) are masked so as not to be affected by the value I am sending. But what I have found is that if I do... L 1 T QW6 This sets bit Q7.0 and NOT Q6.0 as I would have expected (coming froma Rockwell ABbackground). Am I right in thinking that whenever I want to senda decimal value to the output word I must first use CAW to swap the bytes around? Is this the correct approach or is there a better method? Any replies greatly appreciated. Thanks, RandomCoder Splitted from Swaped word.
	Last edited by: O_Moderator at: 10/20/2010 10:15 AM new subject after splitting
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10/19/2010 2:10 PM	Rate (1)
Aret Posts: 2348 Rating: (259)	For output area (supose analog output) - no, you just transfer value to (P)QW. For data excnage with 3rd party device (smart sensor or another PLC) - sometimes you need CAW - in cases when Siemens and this device have different endings. As for 12 bit input - standart way is leftward assignment, i.e. value = XXXX XXXX XXXX 0000. And yes, in your case it would look like QW6= QB6+QB7 = XXXX XXXX + XXXX 000A, but least significant bit (Q7.0) located at "A" letter. So generally it looks like Q6.7 Q6.6 ... Q6.0 Q7.7 Q7.6...Q7.0 etc.
Aret Posts: 2348 Rating: (259)
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10/19/2010 2:47 PM	Rate (0)
RandomCoder Experienced Member Joined: 8/16/2010 Last visit: 1/19/2022 Posts: 50 Rating: (8)	This is obviously something that I need to be very careful about. I did consider swapping my wiring around but it is nice to also be able to read the present value directlyvia the LED's on the module. It is for this reason that I have decided to stick with using the CAW function to swap the byte sequence round to suit the output word address. Kind regards, RandomCoder

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