Siemens Simatic S7-200 System Manual page 94

S7-200 Programmable Controller System Manual
Start and End Conditions for the Receive Instruction
The Receive instruction uses the bits of the receive message control byte (SMB87 or SMB187) to define
the message start and end conditions.
Tip
If there is traffic present on the communications port from other devices when the Receive instruction is
executed, the receive message function could begin receiving a character in the middle of that
character, resulting in a possible parity error and termination of the receive message function. If parity is
not enabled the received message could contain incorrect characters. This situation can occur when the
start condition is specified to be a specific start character or any character, as described in item 2. and
item 6. below.
The Receive instruction supports several message start conditions. Specifying a start condition
involving a break or an idle line detection avoids this problem by forcing the receive message function to
synchronize the start of the message with the start of a character before placing characters into the
message buffer.
The Receive instruction supports several start conditions:
6
1. Idle line detection: The idle line condition is defined as a quiet or idle time on the transmission line.
A receive is started when the communications line has been quiet or idle for the number of
milliseconds specified in SMW90 or SMW190. When the Receive instruction in your program is
executed, the receive message function initiates a search for an idle line condition. If any characters
are received before the idle line time expires, the receive message function ignores those
characters and restarts the idle line timer with the time from SMW90 or SMW190. See Figure 6-10.
After the idle line time expires, the receive message function stores all subsequent characters
received in the message buffer.
The idle line time should always be greater than the time to transmit one character (start bit, data
bits, parity and stop bits) at the specified baud rate. A typical value for the idle line time is three
character times at the specified baud rate.
You use idle line detection as a start condition for binary protocols, protocols where there is not a
particular start character, or when the protocol specifies a minimum time between messages.
Receive instruction is executed:
starts the idle time
Figure 6-10
2. Start character detection: The start character is any character which is used as the first character of
a message. A message is started when the start character specified in SMB88 or SMB188 is
received. The receive message function stores the start character in the receive buffer as the first
character of the message. The receive message function ignores any characters that are received
before the start character. The start character and all characters received after the start character
are stored in the message buffer.
Typically, you use start character detection for ASCII protocols in which all messages start with the
same character.
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Setup: il = 1, sc = 0, bk = 0, SMW90/SMW190 = idle line timeout in milliseconds
Characters
Restarts the idle time
Using Idle Time Detection to Start the Receive Instruction
Setup: il = 0, sc = 1, bk = 0, SMW90/SMW190 = don't care, SMB88/SMB188 = start
character
Characters
First character placed in the
message buffer
Idle time is detected:
starts the Receive Message function