MA-033 Rev. 14.0, 12-Nov-18
Hydran* 201i Instruction Manual
is an industry-standard method used for error detection. If a Modbus slave
device receives a packet in which an error is indicated by the CRC, the slave
device discards the packet, thus preventing any erroneous operations. See
Section J.2.4 below for details on CRC calculation.
• Dead Time: The packet synchronization is achieved with minimum dead time
insertion between packets. In a standard Modbus RTU implementation, a
packet is terminated when no data is received for a period of 3.5-byte
transmission time (about 15 milliseconds at 2,400 bps, 2 milliseconds at
19,200 bps, and 300 microseconds at 115,200 bps). Consequently, the
transmitting device does not allow gaps longer than this interval between
bytes in a given message. Once the dead time has expired without a new byte
transmission, all slaves start listening for a new packet from the master
except for the addressed slave.
Perception software cannot meet the standard 3.5-byte dead
time delay requirement in reception at high speed. For the
Hydran 201Ti firmware, the end-of-frame detection delay is
replaced by a fixed 20-millisecond delay. This implement-
ation limitation may cause a problem on the RS-485 bus
when addressing different slaves in two consecutive
requests. The master must respect the 20-milli-
second dead
time delay following the slave’s response before sending a
new request to another slave. Failure to do so might result in
an exception response or no response from the newly-
addressed slave. This limitation causes no problem when
requests are sent to the same slave.
J.2.4 CRC-16 Algorithm
The CRC-16 algorithm (16-bit Cyclic Redundancy Check) essentially processes the
entire data stream (data bits only; start, stop and parity are ignored) as one continuous
binary number. This number is first shifted left 16 bits and then divided by a
characteristic polynomial term (11000000000000101B). The 16-bit remainder of the
division is appended to the end of the packet, MSB first. The resulting packet including
the CRC, when divided by the same polynomial term at the receiver, gives a zero
remainder if no transmission errors have occurred. This algorithm requires the
characteristic polynomial to be reverse-bit-ordered. The most significant bit of the
characteristic polynomial is dropped, since it does not affect the value of the remainder.
The C programming language implementation of the CRC algorithm can be provided
upon request.
Table J-2 below presents the CRC-16 algorithm. The symbols used are described in
Table J-3 below.
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