Data Field; Crc Check Field; Register Addressing - GE AF-650 GP Design And Installation Manual

RS485 Installation and Set-...
Code Name
3 Illegal data
value
4 Slave device
failure
Table 9.7 Modbus Exception Codes

9.2.2.4 Data Field

The data field is constructed using sets of two hexadecimal
digits, in the range of 00 to FF hexadecimal. These are
made up of one RTU character. The data field of messages
sent from a master to slave device contains additional
information which the slave must use to take the action
defined by the function code. This can include items such
9 9
as coil or register addresses, the quantity of items to be
handled, and the count of actual data bytes in the field.

9.2.2.5 CRC Check Field

Messages include an error-checking field, operating based
on a Cyclic Redundancy Check (CRC) method. The CRC
field checks the contents of the entire message. It is
applied regardless of any parity check method used for the
individual characters of the message. The CRC value is
calculated by the transmitting device, which appends the
CRC as the last field in the message. The receiving device
recalculates a CRC during receipt of the message and
compares the calculated value to the actual value received
in the CRC field. If the two values are unequal, a bus
timeout results. The error-checking field contains a 16-bit
binary value implemented as two 8-bit bytes. When this is
done, the low-order byte of the field is appended first,
followed by the high-order byte. The CRC high-order byte
is the last byte sent in the message.

9.2.3 Register Addressing

In Modbus, all data are organised in coils and holding
registers. Coils hold a single bit, whereas holding registers
hold a 2-byte word (i.e. 16 bits). All data addresses in
Modbus messages are referenced to zero. The first
occurrence of a data item is addressed as item number
zero. For example: The coil known as 'coil 1' in a
90
Meaning
A value contained in the query data field
is not an allowable value for server (or
slave). This indicates a fault in the
structure of the remainder of a complex
request, such as that the implied length is
incorrect. It specifically does NOT mean
that a data item submitted for storage in
a register has a value outside the
expectation of the application program,
since the Modbus protocol is unaware of
the significance of any particular value of
any particular register.
An unrecoverable error occurred while the
server (or slave) was attempting to
perform the requested action.
DET-767c All rights reserved.
programmable controller is addressed as coil 0000 in the
data address field of a Modbus message. Coil 127 decimal
is addressed as coil 007EHEX (126 decimal).
Holding register 40001 is addressed as register 0000 in the
data address field of the message. The function code field
already specifies a 'holding register' operation. Therefore,
the '4XXXX' reference is implicit. Holding register 40108 is
addressed as register 006BHEX (107 decimal).
Coil Description
number
1–16 Frequency converter control word
17–32 Frequency converter speed or set-
point reference Range 0x0 – 0xFFFF
(-200% ... ~200%)
33–48 Frequency converter status word
(see Table 9.10)
49–64 Open loop mode: Frequency
converter output frequency Closed
loop mode: Frequency converter
feedback signal
65 Parameter write control (master to
slave)
0 Parameter changes are written to
= the RAM of the frequency
converter
1 Parameter changes are written to
= the RAM and EEPROM of the
frequency converter.
66– Reserved
65536
Table 9.8 Coil Descriptions
Coil 0
01 Preset reference LSB
02 Preset reference MSB
03 DC brake
04 Coast stop
05 Quick stop
06 Freeze freq.
07 Ramp stop
08 No reset
09 No jog
10 Ramp 1
11 Data not valid
12 Relay 1 off
13 Relay 2 off
14 Set up LSB
15 Set up MSB
16 No reversing
Table 9.9 Frequency Converter Control Word (Drive Profile)
Signal direction
Master to slave
Master to slave
Follower to slave
Follower to slave
Master to slave
1
No DC brake
No coast stop
No quick stop
No freeze freq.
Start
Reset
Jog
Ramp 2
Data valid
Relay 1 on
Relay 2 on
Reversing
MG35M402