Siemens SIMATIC CP 440, Руководство пользователя

Страница: 31 / 172

Basic Principles of Serial Data Transmission
2-13
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Send data
The figure below illustrates a send operation.
The amount of data to
be sent is taken from
the LEN parameter of
the send job.
Waiting for
send job
Send job arrived
Sending user
data
Job processed
Figure 2-4 Sequence of a Send Operation
Receiving Data with the ASCII Driver
In data transmission using the ASCII driver, you can choose between three
different end criteria when data is received. The end criterion defines when a
complete message frame is received. The possible end criteria are as follows:
•
On Expiry of Character Delay Time
The message frame has neither a fixed length nor a defined end-of-text
character; the end of the message is defined by a pause on the line (expiry of
character delay time).
•
On Receipt of One or More End-of-Text Character(s)
The end of the message frame is marked by one or two defined end-of-text
characters.
•
On Receipt of a Fixed Number of Characters
The length of the receive message frames is always identical.