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2/25/2008 9:50 AM | |
Posts: 15 Rating: (0) |
Hi I would like to use a 224XP and an absolute encoder. Is that possible? If so, how do I do? Best regards Budda |
3/6/2008 12:12 AM | |
Posts: 5 Rating: (0) |
Hello Budda, You can use several types of absolute encoders with the 224XP CPU. It depends on the type of output the encoder has and when you need to read the data from the encoder. Some of the common output types that will not work are Serial Synchronous Interface (SSI) orTTL. You will need an encoder that supports one of the following output types: 1)RS232 or RS485 serial interface - You will have to write a Freeport program to read the data into the S7-200. I do not know of a protocol library for any particular encoder that is already written. 2) 24 vdc digital outputs - This will require one input on the S7-200 for each bit of resolution you want from the encoder. If you have a 12 bit absolute encoder then you will have to use up to 12 digital inputs to read the data. If you need less resolution then you do not need to use all the inputs. The data format from the absolute encoder is normally Gray code but can be natural binary. When using Gray code you will have to convert the input values to natural binary before it can be used. One site that explains how to do this is: http://www.faqs.org/faqs/ai-faq/genetic/part6/section-1.html You also have to consider when you will be reading the value. If the absolute encoder value is not changing when you read the value (i.e. it is not moving) then it does not make any difference. However, if it is moving then you should use a serial interface. If you have to use the digital outputs then you can only use 8 inputs and these must all be in the same byte (For example: IB0). The S7-200 CPU only has data consistence on a byte boundary when reading the inputs. It is possible for a system or user interrupt to occur between when the system reads one byte and before it reads the second byte. Therefore, this might create an incorrect value if you use more that 8 bits. |
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