GE GFK-0804B manual Message 03 Read Registers Format

C

Message (03): Read Registers

Format:

Address Func 03

Starting

Register No.

Number of Registers

Error Check

Hi Lo Hi Lo

Query

Address Func 03

Byte Count

Data

First

Register

Error Check

Hi Lo Hi Lo

Normal Response

Query:

HAn address of 0 is not allowed as this request cannot be a broadcast request.

HThe function code is equal to 03.

HThe starting register number is two bytes in length. The starting register number may be any value less than the highest register number available in the Micro PLC. It is equal to one less than the number of the first register returned in the normal response to this request.

HThe number of registers value is two bytes in length. It must contain a value from 1 to 125 inclusive. The sum of the starting register value and the number of registers value must be less than or equal to the highest register number available in the Micro PLC. The high order byte of the starting register number and number of registers fields is sent as the first byte in each of these fields. The low order byte is the second byte in each of these fields.

Response:

HThe byte count is a binary number from 2 to 250 inclusive. It is the number of bytes in the normal response following the byte count and preceding the error check. Note that the byte count is equal to two times the number of registers returned in the response. A maximum of 250 bytes (125) registers is set so that the entire response can fit into one 256 byte data block.

HThe registers are returned in the data field in order of number with the lowest number register in the first two bytes and the highest number register in the last two bytes of the data field. The number of the first register in the data field is equal to the starting register number plus one. The high order byte is sent before the low order byte of each register.

GFK±0804B Appendix C RTU Protocol C-11