Siemens SIMATIC S5-115U, User Manual

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Communications Capabilities S5-115U Manual
Data traffic is bidirectional:
• Send
The ASCII driver processes data in the user memory (e.g. the contents of a data block) and
outputs it at the second interface.
• Receive
An I/O device sends data in ASCII code to the second interface. The ASCII driver processes the
data and stores it in internal RAM.
Data are only received every 100 ms. Please take this into account for shorter CPU cycle times.
The two memory areas in internal RAM in which the send and receive data are stored are called
the Send mailbox (SMB) and the Receive mailbox (RMB).
The data can be stored either in a data block or the flag area. You must enter the relevant
information in the parameter block (see Table 12-13).
Send and Receive mailboxes have the following properties:
• 1024 bytes of input buffer are available in all modes.
• In modes where characters are interpreted when received (e.g. XON, XOFF), the ASCII driver
can still receive data or message frames even if it has already sent XOFF to the
communications partner. In this case, the ASCII driver receives data until the input buffer is
full, or it receives message frames until the maximum possible number of frames has been
reached.
Example of "borderline case":
If a received message frame is 1024 bytes long and the ASCII driver then sends XOFF, there is
no way of buffering characters received from the communications parnter after sending
XOFF.
• In mode 1, 7 or 8 (see Section 12.4.3), you must enter in the first word of the send mailbox the
number of bytes of data to be sent.
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EWA 4NEB 811 6130-02b