Siemens SIMATIC S7-200 System Manual page 105

The Receive instruction supports several ways to terminate a message. The message can be
terminated on one or a combination of the following:
1. End character detection: The end character is any character which is used to denote the
end of the message. After finding the start condition, the Receive instruction checks each
character received to see if it matches the end character. When the end character is
received, it is placed in the message buffer and the receive is terminated.
Typically, you use end character detection with ASCII protocols where every message ends
with a specific character. You can use end character detection in combination with the
intercharacter timer, the message timer or the maximum character count to terminate a
message.
Setup: ec = 1, SMB89/SMB189 = end character
2. Intercharacter timer: The intercharacter time is the time measured from the end of one
character (the stop bit) to the end of the next character (the stop bit). If the time between
characters (including the second character) exceeds the number of milliseconds specified
in SMW92 or SMW192, the receive message function is terminated. The intercharacter
timer is restarted on each character received. See Figure 6-12.
You can use the intercharacter timer to terminate a message for protocols which do not
have a specific end-of-message character. This timer must be set to a value greater than
one character time at the selected baud rate since this timer always includes the time to
receive one entire character (start bit, data bits, parity and stop bits).
You can use the intercharacter timer in combination with the end character detection and
the maximum character count to terminate a message.
Setup: c/m = 0, tmr = 1, SMW92/SMW192 = timeout in milliseconds
Characters
Restarts the intercharacter
timer
Figure 6-12 Using the Intercharacter Timer to Terminate the Receive Instruction
3. Message timer: The message timer terminates a message at a specified time after the start
of the message. The message timer starts as soon as the start condition(s) for the receive
message function have been met. The message timer expires when the number of
milliseconds specified in SMW92 or SMW192 have passed. See Figure 6-13.
Typically, you use a message timer when the communications devices cannot guarantee
that there will not be time gaps between characters or when operating over modems. For
modems, you can use a message timer to specify a maximum time allowed to receive the
message after the message has started. A typical value for a message timer would be
about 1.5 times the time required to receive the longest possible message at the selected
baud rate.
You can use the message timer in combination with the end character detection and the
maximum character count to terminate a message.
Setup: c/m = 1, tmr = 1, SMW92/SMW192 = timeout in milliseconds
S7-200 Instruction Set
Characters
The intercharacter timer expires:
Terminates the message and generates the
Receive Message interrupt
Chapter 6
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