Start/Stop Field; Address Field; Function Field - GE AF-650 GP Design And Installation Manual

RS485 Installation and Set-...
during 'silent' intervals. When the first field (the address
field) is received, each frequency converter or device
decodes it to determine which device is being addressed.
Modbus RTU messages addressed to zero are broadcast
messages. No response is permitted for broadcast
messages. A typical message frame is shown in Table 9.4.
Start Address Function
T1-T2-T3- 8 bits 8 bits
T4
Table 9.4 Typical Modbus RTU Message Structure

9.2.2.1 Start/Stop Field

Messages start with a silent period of at least 3.5 character
intervals. This is implemented as a multiple of character
intervals at the selected network baud rate (shown as Start
T1-T2-T3-T4). The first field to be transmitted is the device
address. Following the last transmitted character, a similar
period of at least 3.5 character intervals marks the end of
the message. A new message can begin after this period.
The entire message frame must be transmitted as a
continuous stream. If a silent period of more than 1.5
character intervals occurs before completion of the frame,
the receiving device flushes the incomplete message and
assumes that the next byte is the address field of a new
message. Similarly, if a new message begins prior to 3.5
character intervals after a previous message, the receiving
device will consider it a continuation of the previous
message. This causes a timeout (no response from the
slave), since the value in the final CRC field is not valid for
the combined messages.

9.2.2.2 Address Field

The address field of a message frame contains 8 bits. Valid
slave device addresses are in the range of 0–247 decimal.
The individual slave devices are assigned addresses in the
range of 1–247. (0 is reserved for broadcast mode, which
all slaves recognize.) A master addresses a slave by placing
the slave address in the address field of the message.
When the slave sends its response, it places its own
address in this address field to let the master know which
slave is responding.

9.2.2.3 Function Field

The function field of a message frame contains 8 bits. Valid
codes are in the range of 1-FF. Function fields are used to
send messages between master and slave. When a
message is sent from a master to a slave device, the
function code field tells the slave what kind of action to
perform. When the slave responds to the master, it uses
the function code field to indicate either a normal (error-
free) response, or that some kind of error occurred (called
MG35M402
AF-650 GP™ Design and Installation Guide
Data CRC End
check
N x 8 bits 16 bits T1-T2-T3-
T4
DET-767c All rights reserved.
an exception response). For a normal response, the slave
simply echoes the original function code. For an exception
response, the slave returns a code that is equivalent to the
original function code with its most significant bit set to
logic 1. In addition, the slave places a unique code into the
data field of the response message. This tells the master
what kind of error occurred, or the reason for the
exception.
Function
Read coils
Read holding registers
Write single coil
Write single register
Write multiple coils
Write multiple registers
Get comm. event counter
Report slave ID
Table 9.5
Function Function Subfunctio
code n code
1
2
10
11
Diagnostics 8
12
13
14
Table 9.6
Code Name Meaning
1 Illegal The function code received in the query is
function not an allowable action for the server (or
slave). This may be because the function
code is only applicable to newer devices,
and was not implemented in the unit
selected. It could also indicate that the
server (or slave) is in the wrong state to
process a request of this type, for example
because it is not configured and is being
asked to return register values.
2 Illegal data The data address received in the query is
address not an allowable address for the server (or
slave). More specifically, the combination
of reference number and transfer length is
invalid. For a controller with 100 registers,
a request with offset 96 and length 4
would succeed, a request with offset 96
and length 5 generates exception 02.
Function code
1 hex
3 hex
5 hex
6 hex
F hex
10 hex
B hex
11 hex
Subfunction
Restart communication
Return diagnostic register
Clear counters and
diagnostic register
Return bus message count
Return bus communi-
cation error count
Return bus exception error
count
Return slave message
count
89
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