Chapter 6. Serial I/O, SNP & RTU Protocols230 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222ADInformation FieldsAll message fields, other than the Station Address field, Function Code field, and Error Check field arecalled, generically, information fields. Information fields contain additional information required tospecify or respond to a requested function. Different types of messages have different types or numbersof information fields. (Details on information fields for each message type and function code are found inRTU Message Descriptions. Some messages (Message 07 Query and Message 17 Query) do not haveinformation fields.ExamplesAs shown in the following figure, the information fields for message READ OUTPUT TABLE (01) Queryconsist of the Starting Point No. field and Number of Points field. The information fields for messageREAD OUTPUT TABLE (01) Response consist of the Byte Count field and Data field.Address StartingPoint No.Number ofPointsErrorCheckFunc01Information FieldsQueryHi HiLo LoMessage (01)Read Output TableAddress Data ErrorCheckFunc01Information FieldsNormal ResponseByteCountFigure 50: RTU Read Output Table ExampleSome information fields include entries for the range of data to be accessed in the RTU slave.Note: Data addresses are 0-based. This means you will need to subtract 1 from the actual addresswhen specifying it in the RTU message. For message (01) READ OUTPUT TABLE Query, usedin the example above, you would specify a starting data address in the Starting Point No.field. To specify %Q0001 as the starting address, you would place the address %Q0000 inthis field. Also, the value placed in the Number of Points field determines how many %Q bitsare read, starting with address %Q0001. For example:▪ Starting Point No. field = %Q0007, so the starting address is %Q0008.▪ Number of Points field = 16 (0010h), so addresses %Q0008 through %Q0023 will be read.Error Check FieldThe Error Check field is two bytes in length and contains a cyclic redundancy check (CRC-16) code. Itsvalue is a function of the contents of the station Address, Function code, and Information field. Thedetails of generating the CRC-16 code are described in Cyclic Redundancy Check (CRC). Note that theInformation field is variable in length. To properly generate the CRC-16 code, the length of frame mustbe determined. To calculate the length of a frame for each of the defined function codes, see Calculatingthe Length of Frame.
