VP-1405A Advanced Power
Chapter 6 - Protocol and Communication
V-PM1000 Series, Veesta Power Module Product Series
48V Power Supply & Charger Instructions & User's Manual
Page 34 of 46
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information varied with the type and capacity of slave associated with
the slave.
In the VP-1405A maximum of N = 95 bytes. You should observe this
limitation when you are calling to read out data or setting data by
different function codes.
6.4.4
Error Check filed
The field allows the master and slave devices to check a message
for errors in transmission. Sometimes, because of electrical noise or
other interference, a message may be changed slightly while it is on
its way from one unit to another. The error checking assures that the
slave or master does not react to messages that have changed
during transmission. This increases the safety and the efficiency of
the system. The error check uses a CRC – 16 check methods.
Note: The sending sequence is always the same - address, function
code, data, and error check - relative to the direction
6.4.5
Error Detection
The RTU version of Modbus® includes a two byte CRC-16 (16 bit
cyclic redundancy check) with every transmission. The CRC-16
algorithm essentially treats the entire data stream (data bits only;
start, stop and parity ignored) as one continuous binary number.
This number is first shifted left 16 bits and then divided by a
characteristic polynomial (11000000000000101B). The 16-bit
remainder of the division is appended to the end of the transmission,
MSB byte first. The resulting message including CRC, when divided
by the same polynomial at the receiver will give a zero remainder if
no transmission errors have occurred.
If a VP-1405A Modbus® slave device receives a transmission in
which an error is indicated by the CRC-16 calculation, the slave
device will not respond to the transmission.
A CRC-16 error indicates than one or more bytes of the transmission
were received incorrectly and thus the entire transmission should be
ignored in order to avoid the VP-1405A performing any incorrect
operation.
The CRC-16 calculation is an industry standard methodized used for
error detection. An algorithm is included here to assist programmers
in situations where no standard CRC-16 calculation routines are
available.
