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1.3. Address field
A frame is started with an address field, which consists of 8
bits (taken from 1 to 255). These bits indicate the address of the
slave device designated by the user, which will receive data from
the host it connects to. The address of each slave device must
be unique, and only the addressed slave device responds to
the query containing the address. When the slave sends back a
response, the slave address data in the response will tell the host
which slave device it is communicating with.
1.4. Function field
The function field code tells which function is to performed by the
addressed slave device. Table 1–1 lists all function codes, their
meanings and initial functions.
Table 1-1 Function Code
Code
Defi nition
Behaviors
03(03H)
Read data register
Get the current binary value of one or more registers
06(06H)
Preset single register Place a specifi c binary value in a register
16(10H)
Preset multiple
registers
Place specifi c binary values in a series of registers
1.5. Data field
A data field contains the data required by the slave to perform a
specific function or the data collected when the slave responds to
a query. These data may be numerical values, reference addresses
or limiting values. For example, a function field code tells the slave
to read a register, while a data field needs to specify which reg-
ister to start and how many data to read. The embedded address
and data are subject to slave types and the capabilities among
slave devices.
1.6. Error check field
An error check field allows the host and slave to check for errors
during transmission. In some cases, due to electrical noise and
other interferences, a set of data may be changed when trans-
mitted from one device to another. Error checking can ensure
the host or slave not to respond to the data changed during the
transmission, which can improve system safety and efficiency. The
error check adopts a 16-bit cyclic redundancy method.
1.7. Error detection
The cyclic redundancy check (CRC) field occupies two bytes and
contains a 16-bit binary value. The CRC value is computed by the
transmitting device and then appended to the data frame. The
receiving device recalculates the CRC value when receiving the
data, and compares it with the value in the received CRC field. If
the two values are not equal, an error has occurred.
During CRC computing, first preset a 16-bit register to all 1s; then
calculate the 8-bit byte in the data frame continuously with the
current value of this register; only the 8 data bits of each byte are
involved in generating a CRC. The start bit, stop bit, and parity bit
that may be used do not apply to CRC.
During generating a CRC, XOR each 8-bit byte with contents in
the register, shift the result to the low bit, and supplement the
high bit with "0". Then shift out and check the least significant bit
(LSB). If the LSB is 1, this register will perform an exclusive OR
operation with a preset fixed value. If 0, then no action will be
taken.
Repeat the above process until eight shift operations are per-
formed. When the last bit (the 8th bit) is shifted, XOR the next
8-bit byte with the current value of the register. Perform another
eight shift XOR operations described above. When all the bytes
in the data frame are processed, the final value generated is the
CRC value.
To generate a CRC, follow below steps:
(1) Preset a 16-bit register as 0FFFFH (all 1s), called CRC register;
(2) Perform an XOR computing of the first 8-bit byte in the data
frame with the low byte in the CRC register, and store the
result in the CRC register;
(3) Shift the CRC register one bit to the right, fill the most sig-
nificant bit (MSB) with 0, shift out the least significant bit and
check;
(4) If the LSB is 0: repeat the third step (perform next shift);
If the LSB is 1: perform an XOR computing of the CRC regis-
ter with a preset fixed value (0A001H);
(5) Repeat Step 3 and 4 until eight shifts are performed. In this
way, eight bits will be processed completely;
(6) Repeat Step 2 to Step 5 to process the next eight bits until all
bytes are processed;
(7) The final value generated in the CRC register is the CRC value.
Instruction Leafl et
Effective September 2022
IZM6 Series Air Circuit Breaker
Operating Instruction Leafl et