GE Industrial Solutions ASTAT BP - Manual (Page 81)

ASTAT BP User Manual
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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-BP) and the Response transmission from the Slave to the Master:
"sync": Silent Interval for at least 3.5 character times
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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-BP Slave Number (1..247) on the serial link. The ASTAT-BP 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-BP.
A.5. Function
The Function code informs the ASTAT-BP 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.
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When a parity bit is used each character is 11 bits. Therefore the silent interval is 3.5 * 11 / baud rate.