Siemens SIMATIC TIWAY 1 User Manual page 183

USING THE NON-INTELLIGENT TERMINAL PROTOCOL
A.2 NITP Protocol Operation
Setting Position 1 of Dipswitch 1 up selects NITP as the communication protocol
between the host computer and the adapter. Messages in the form of adapter
commands are composed and framed by the host computer and transmitted to
the adapter. The adapter receives the message, processes the command,
composes and frames the response, and transmits the response to the host
computer. This command/response sequence ensures that the messages are
always delivered in their intended order.
After the semicolon in the response, the adapter will also send the ASCII
carriage return and line feed characters, 0D and 0A respectively, so that
messages are scrolled on non-intelligent terminals.
When the adapter is powered-up, it enters the "receive" state and waits for the
host computer to transmit the first command. When a message is received, the
adapter will first verify the NITP frame by verifying the length field and the
ECC. If any errors are found, the adapter will format and transmit the ERROR
RESPONSE. Typical errors are described below in Paragraph 5.2 . If there are
no NITP frame errors, the adapter will verify and process the command code
and its parameters. After the command is processed, the adapter will format
and transmit the response to the host computer.
A–7 UNILINK HOST ADAPTER
USER MANUAL