6
2.5 Data Field
The data field contains information needed by the instrument to perform a specific function, or data
collected by the instrument in response to a query.
IMPORTANT
Fields composed of two bytes are sent in the order high byte first, low byte second.
2.6 Error Check Field
The error check field contains the Cyclical Redundancy Check (CRC) word. The start of the message
is ignored in calculating the CRC. The CRC-16 error check sequence is implemented as described in
the following paragraphs.
The message (data bits only, disregarding start/stop and optional parity bits) is considered one
continuous binary number whose most significant bit (MSB) is transmitted first. The message is pre-
multiplied by x
16
(shifted left 16 bits), and then divided by x
16
+ x
15
+ x
2
+ 1 expressed as a binary
number (11000000000000101). The integer quotient digits are ignored and the 16-bit remainder
(initialized to all ones at the start to avoid the case of all zeros being an accepted message) is
appended to the message (MSB first) as the two CRC check bytes. The resulting message including
CRC, when divided by the same polynomial (x
16
+ x
15
+ x
2
+ 1) at the receiver will give a zero
remainder if no errors have occurred. (The receiving unit recalculates the CRC and compares it to the
transmitted CRC). All arithmetic is performed modulo two (no carries).
The device used to serialize the data for transmission will send the conventional LSB or right-most bit
of each character first. In generating the CRC, the first bit transmitted is defined as the MSB of the
dividend. For convenience, and since there are no carries used in the arithmetic, let's assume while
computing the CRC that the MSB is on the right. To be consistent, the bit order of the generating
polynomial must be reversed. The MSB of the polynomial is dropped since it affects only the quotient
and not the remainder. This yields 1010 0000 0000 0001 (Hex A001). Note that this reversal of the bit
order will have no effect whatever on the interpretation or bit order of characters external to the CRC
calculations.
The step by step procedure to form the CRC-16 check bytes is as follows:
1.
Load a 16-bit register with all 1's.
2.
Exclusive OR the first 8-bit byte with the low order byte of the 16-bit register, putting the result in the 16-bit
register.
3.
Shift the 16-bit register one bit to the right.
4a. If the bit shifted out to the right (flag) is one, exclusive OR the generating polynomial 1010 000 000 0001
with the 16-bit register.
4b. If the bit shifted out to the right is zero, return to step 3.
5.
Repeat steps 3 and 4 until 8 shifts have been performed.
6.
Exclusive OR the next 8-bit byte with the 16-bit register.
7.
Repeat step 3 through 6 until all bytes of the message have been exclusive ORed with the 16-bit register
and shifted 8 times.
8.
When the 16-bit CRC is transmitted in the message, the low order byte will be transmitted first, followed by
the high order byte.
For detailed information about CRC calculation, refer to the Modbus Protocol Reference Guide.
