Basler Electric BE1-851 Modbus Protocol - Manual (Page 14)

1-6 BE1-851 Modbus
™ Protocol
9289900992 Rev F
Query
Data will cease to be written if any of the following exceptions occur:
• Queries to write to Read only registers result in an error response with Exception Code of “Illegal
Data Address.”
• A query to write a value that is not allowed (out of range) to a register results in an error response
with Exception Code of “Illegal Data Value.”
Device Address
Function Code =
06 (hex)
Address Hi
Address Lo
Data Hi
Data Lo
CRC Hi error check
CRC Lo error check
Response
The response message echoes the Query message after the register has been altered.
DATA FORMATS
BE1-851 data varies from one to four bytes in length. Single byte data resides in the holding register
least-significant byte with the most-significant byte set to zero. Floating-point data and long integer data
(each 32-bits in length) place the two most-significant bytes in the higher holding register address of the
associated register pair.
Floating Point Data Format (FP)
The Modbus
™ floating point data format uses two consecutive holding registers to represent a data value.
The first register contains the low-order 16 bits of the following 32-bit format:
• MSB is the sign bit for the floating-point value (0 = positive).
• The next 8 bits are the exponent biased by 127 decimal.
• The 23 LSBs comprise the normalized mantissa. The most-significant bit of the mantissa is always
assumed to be 1 and is not explicitly stored, yielding an effective precision of 24 bits.
The value of the floating-point number is obtained by multiplying the binary mantissa times two raised to
the power of the unbiased exponent. The assumed bit of the binary mantissa has the value of 1.0, with
the remaining 23 bits providing a fractional value. Table 1-3 shows the floating-point format.
Table 1-3. Floating Point Format
Sign Exponent + 127 Mantissa
1 bit 8 bits 23 bits
The floating-point format allows for values ranging from approximately 8.43X10
-37
to 3.38X10
38
. A floating-
point value of all zeroes is the value zero. A floating-point value of all ones (not a number) signifies a
value currently not applicable or disabled.
Example: The value 95,800 represented in floating point format is hexadecimal 47BB1C00. This number
will read from two consecutive holding registers as follows:
Holding Register Value
K (Hi Byte)
hex 1C
K (Lo Byte) hex 00
K+1 (Hi Byte) hex 47
K+1 (Lo Byte) hex BB
The same byte alignments are required to write.