Siemens SIMATIC S7-200 System Manual page 104

S7-200 Programmable Controller System Manual
3. Idle line and start character: The Receive instruction can start a message with the
combination of an idle line and a start character. When the Receive instruction is executed,
the receive message function searches for an idle line condition. After finding the idle line
condition, the receive message function looks for the specified start character. If any
character but the start character is received, the receive message function restarts the
search for an idle line condition. All characters received before the idle line condition has
been satisfied and before the start character has been received are ignored. The start
character is placed in the message buffer along with all subsequent characters.
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.
Typically, you use this type of start condition when there is a protocol that specifies a
minimum time between messages, and the first character of the message is an address or
something which specifies a particular device. This is most useful when implementing a
protocol where there are multiple devices on the communications link. In this case the
Receive instruction triggers an interrupt only when a message is received for the specific
address or devices specified by the start character.
Setup:
character
4. Break detection: A break is indicated when the received data is held to a zero value for a
time greater than a full character transmission time. A full character transmission time is
defined as the total time of the start, data, parity and stop bits. If the Receive instruction is
configured to start a message on receiving a break condition, any characters received after
the break condition are placed in the message buffer. Any characters received before the
break condition are ignored.
Typically, you use break detection as a start condition only when a protocol requires it.
Setup:
don't care
5. Break and a start character: The Receive instruction can be configured to start receiving
characters after receiving a break condition, and then a specific start character, in that
sequence. After the break condition, the receive message function looks for the specified
start character. If any character but the start character is received, the receive message
function restarts the search for an break condition. All characters received before the break
condition has been satisfied and before the start character has been received are ignored.
The start character is placed in the message buffer along with all subsequent characters.
Setup:
6. Any character: The Receive instruction can be configured to immediately start receiving any
and all characters and placing them in the message buffer. This is a special case of the idle
line detection. In this case the idle line time (SMW90 or SMW190) is set to zero. This forces
the Receive instruction to begin receiving characters immediately upon execution.
Setup:
Starting a message on any character allows the message timer to be used to time out the
receiving of a message. This is useful in cases where Freeport is used to implement the
master or host portion of a protocol and there is a need to time out if no response is
received from a slave device within a specified amount of time. The message timer starts
when the Receive instruction executes because the idle line time was set to zero. The
message timer times out and terminates the receive message function if no other end
condition is satisfied.
Setup:
90
il = 1, sc = 1, bk = 0, SMW90/SMW190 > 0, SMB88/SMB188 = start
il = 0, sc = 0, bk = 1, SMW90/SMW190 = don't care, SMB88/SMB188 =
il = 0, sc = 1, bk = 1, SMW90/SMW190 = don't care,
SMB88/SMB188 = start character
il = 1, sc = 0, bk = 0, SMW90/SMW190 = 0, SMB88/SMB188 = don't care
il = 1, sc = 0, bk = 0, SMW90/SMW190 = 0, SMB88/SMB188 = don't care
c/m = 1, tmr = 1, SMW92 = message timeout in milliseconds