Chapter 6. Serial I/O, SNP & RTU ProtocolsGFK-2222AD April 2018 251Message (67): Read Scratch Pad MemoryFormat:Address StartingByte No.Number ofBytesErrorCheckFunc67QueryAddress Data ErrorCheckFunc67Normal ResponseByteCountFigure 63: RTU Read Scratch Pad Memory Message FormatQuery:▪ An Address of 0 is not allowed as this cannot be a broadcast request.▪ The Function Code is equal to 67.▪ The Starting Byte Number is two bytes in length and may be any value less than or equal to thehighest scratch pad memory address available in the attached CPU as indicated in the table below.The Starting Byte Number is equal to the address of the first scratch pad memory byte returned inthe normal response to this request.▪ The Number of Bytes value is two bytes in length. It specifies the number of scratch pad memorylocations (bytes) returned in the normal response. The sum of the Starting Byte Number and theNumber of Bytes values must be less than two plus the highest scratch pad memory addressavailable in the attached CPU. The high order byte of the Starting Byte Number and Number of Bytesfields is sent as the first byte in each of these fields. The low order byte is the second byte in each ofthe fields.Response:▪ The Byte Count is a binary number from 1 to 256 (0 = 256). It is the number of bytes in the Data fieldof the normal response.▪ The Data field contains the contents of the scratch pad memory requested by the query. The scratchpad memory bytes are sent in order of address. The contents of the scratch pad memory byte whoseaddress is equal to the Starting Byte Number is sent in the first byte of the Data field. The contentsof the scratch pad memory byte whose address is equal to one less than the sum of the starting bytenumber and number of bytes values is sent in the last byte of the Data field.
