
Modbus Commands
2-2
SIMATIC TI505/TI500 MODNIM User Manual
2.1 Modbus Protocol Overview — ASCII Transmission Mode
This
chapter examines the Modbus protocol for both ASCII and RTU modes
of transmission. The MODNIM-supported functions are then described in
detail, with examples of requests and responses for each function code.
Each frame in the ASCII transmission mode begins with a colon (
: ) and
ends with CR LF (Carriage Return/Line Feed), shown in Figure 2-1.
2
characters
n x 2 char
2 char
Address Function Data LRCStart EOF
2 characters
CR LF
Ready
Figure 2-1 ASCII
T
ransmission Frame
Each of the fields in the transmission frame is described in the paragraphs
below.
The Start field is a colon (: = 3A hex), and marks the start of the message.
This is the first character transmitted.
The Address field identifies the node to which this message is being sent
and is a number in the range of 1 to 247. Each node on any one network
must have a unique address. Only those nodes addressed will respond
unless the broadcast address 0 is used. In that case, each slave will read
and act on the message but not respond.
The Function field is a 2-character (16-bit) code that determines the action
that a slave takes when receiving the message. The function codes
supported by the MODNIM are summarized in T
able 2-1 and described in
more detail with examples in section 2.4.
Modbus Pr
otocol
The ASCII
T
ransmission Frame
The Start Field
The Addr
ess Field
The Function Field
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