Unitronics UMI-B1 UL Series Operation Manual page 115

UMI-B1 UL Series Inverter
CMD
DATA (N-1)
...
DATA (0)
CRC CHK low bit
CRC CHK high bit
END
7.2.2.2 RTU communication frame error checkout
Various factors (such as electromagnetic interference) may cause error in the data
transmission. For example, if the sending message is logic "1", A-B potential difference
on RS485 should be 6V, but in reality, it may be -6V because of electromagnetic
interference, and then the other devices take the sent message as logic "0". If there is no
error checkout, the receiving devices will not find the message is wrong and they may
give incorrect response which cause serious result. So the checkout is essential to the
message.
The theme of checkout is that: the sender calculate the sending data according to a
fixed formula, and then send the result with the message. When the receiver gets this
message, they will calculate anther result according to the same method and compare it
with the sending one. If two results are the same, the message is correct. If no, the
message is incorrect.
The error checkout of the frame can be divided into two parts: the bit checkout of the
byte and the whole data checkout of the frame (CRC check).
Bit checkout of the byte
The user can select different bit checkouts or non-checkout, which impacts the check bit
setting of each byte.
The definition of even checkout: add an even check bit before the data transmission to
illustrate the number of "1" in the data transmission is odd number or even number.
When it is even, the check byte is "0", otherwise, the check byte is "1". This method is
used to stabilize the parity of the data.
The definition of odd checkout: add an odd check bit before the data transmission to
illustrate the number of "1" in the data transmission is odd number or even number.
When it is odd, the check byte is "0", otherwise, the check byte is "1". This method is
used to stabilize the parity of the data.
For example, when transmitting "11001110", there are five "1" in the data. If the even
checkout is applied, the even check bit is "1"; if the odd checkout is applied; the odd
check bit is "0". The even and odd check bit is calculated on the check bit position of the
frame. And the receiving devices also carry out even and odd checkout. If the parity of
03H: read slave parameters
06H: write slave parameters
The data of 2 x N bytes are the main content of the
communication as well as the core of data exchanging
Detection value: CRC (16 BIT)
T1-T2-T3-T4 (transmission time of 3.5 bytes)
113
Communication protocol