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2. BASIC STRUCTURE OF THE SERIAL LINK FRAME
Modbus RTU frame have the same principal structure for both the "Query" transmission from the Master
to the Slave (RVS-AXO) and the Response transmission from the Slave to the Master.
"Sync": Silent time of at least 3.5 character (3.5 * 11 bit times).
Byte 1:
Serial Link No. (= Slave Address)
(1...247)
Byte 2:
Function
(1, 2, 3,4,5,6,8,15 &16 are supported)
Byte 3: Data Bytes
($XX)
.
.
($XX)
.
.
($XX)
Byte n-1: CRC_Low
($XX)
Byte n: CRC_High
($XX)
"Sync": Silent time of at least 3.5 character (3.5 * 11 bit times).
SYNC (Silent Interval)
In RTU mode messages
“synchronize" by a "Silent Interval" of more than 3.5 character times. This silent
interval separates between transmission frames.
The entire frame must be transmitted as a continuous stream. A silent time of more than 3.5 character
times during frame transmission will cause the receiving device to ignore the incomplete frame. Next byte
will be assumed as the Serial Link No. of the next frame.
Same result of ignoring the frame can occur if a second message is transmitted before 3.5 character
times from the end of the previous one. This will cause the receiving device to consider it as a continuation
of the first frame, resulting with CRC error.
SERIAL LINK NO. (SLAVE ADDRESS)
Contains RVS-AXO Slave Number (1-127) on the serial link. The RVS-AXO default value is 1. Serial Link
No. is used as the first byte in both the "Query" transmission from Master to Slave and in Response
transmission from Slave to Master.
FUNCTION
The Function code informs the RVS-AXO what is the requested action to take. In normal cases, Function
is used as the second byte in both the "Query" transmission from Master to Slave and in Response
transmission from Slave to Master.
