CTI 2572 Technical Overview Manual
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CHAPTER 6. PLC COMMAND INTERFACE
6.1 Non Intelligent Terminal Protocol
As discussed in previous sections of this manual, application programs communicate with the
PLC by sending messages to the 2572 encapsulated in the TCP/IP protocol. The most common
message format used by SIMATIC TI505 series PLCs is the Non Intelligent Terminal Protocol.
Originally developed for programming units and simple ASCII terminals, NITP is widely used
by most products that access the programming port of the PLC, including PCs running TISOFT.
NITP messages use 7 bit ASCII character format with odd parity. Data is represented in ASCII
hexadecimal format, where a byte is represented as two characters. Although this is less efficient
than a straight binary protocol, use of ASCII Hex frees the programmer from having to
understand how data is physically represented in the PLC. Using NITP protocol, up to 15 words
can be transferred between the external device and the PLC in one message. The beginning
delimiter is a colon (ASCII 3A) and the ending delimiter is a semicolon (ASCII 3B). The
format of the NITP message is shown below:
Note that the message body contains a command (CMD) and up to 15 words (W1-W15). The
command is an instruction to the PLC (such as Read V Memory). The words may represent
addresses or actual data (depending upon the command and whether this is a request or a
response).
Beginning
Delimiter
Character
Count
Message Body ECC
Message
Terminator
1 Byte
1 Byte 64 Bytes Maximum
4 bytes 2 bytes
C
M
D
W
1
W
2
W
3
W
4
W
5
W
6
W
7
W
8
W
9
W
10
W
11
W
13
W
14
W
15
72 Bytes Maximum
Figure 15 NITP Message Format
