GE Industrial Solutions ASTAT XT - Manual (Page 87)

87 • Appendix A - MODBUS RTU Protocol
A.2. Basic Structure of the Serial Link Frame
The Modbus RTU frame has the same principal structure for both the "Query" transmission from the Master to the
Slave (ASTAT-XT) and the Response transmission from the Slave to the Master:
"sync":
Silent Interval for at least 3.5 character times 44
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 Interval for at least 3.5 characters
A.3. SYNC (Silent Interval)
In RTU mode, messages use a "Silent Interval" more than 3.5 characters to "synchronize". This silent interval
separates transmission frames.
The entire frame must be transmitted as a continuous stream. A silent interval of more than 3.5 characters during
frame transmission will cause the receiving device to ignore the incomplete frame. The next byte is assumed to be
the Serial Link No. of the next frame.
Ignoring the frame can also occur if a second message is transmitted less than 3.5 characters from the end of the
previous frame. This causes the receiving device to consider that frame to be a continuation of the first frame, thus
resulting in a CRC error.
A.4. Serial Link No. (Slave Address)
This contains the ASTAT-XT Slave Number (1..247) on the serial link. The ASTAT-XT default value is 248, which is the
communication OFF condition. 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.
Note: Address 0, which is normally used for broadcast transmissions is not supported by the ASTAT-XT.
A.5. Function
The Function code informs the ASTAT-XT what action is requested. In most 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.
A.6. List of Functions Supported By The ASTAT-XT
Function Modbus
Name Use in ASTAT-XT
01
Read Coil Status Read discrete commands status.
02 Read Input Status Read discrete inputs status.
44 When a parity bit is used each character is 11 bits. Therefore the silent interval is 3.5 * 11 / baud rate.