Function Block FB20
FB20 has the following tasks:
• Sending a “sign of life” keep-alive message to the PICO controller
• Writing configuration data messages parallel to keep-alive messages (mutual interlocking and waiting until message X
has received response X before a new message is sent)
• Handling the transaction number (increment, checking the response TAN)
• Switching/converting to the proper formats (REAL/INT) for the written/read values
• Providing information as to whether or not the PDA/PICO controller is available (PDA-available flag)
A time base is provided via FB5 (toggle bit). This is used to cyclically send a keep-alive request and to
simultaneously start a timeout timer. The timeout timer is restarted every time a request is sent. The keep-alive
request is necessary so that the PICO controller can also recognize an existing connection to the PLC and react
accordingly during an interruption. In this manner, local operation is allowed again at any rate in the event of an
interrupted PLC connection (even if Local Service is not set).
While the timeout timer is running, a check is performed to determine whether or not the response from the PICO
controller has the same transaction number as that which was sent. If no response with the same transaction
number is received within the timeout time, the PDA-available flag is reset. The timeout is set to 7 seconds because
the longest commands (Store EE/Flash for all four channels) lasts approximately 6 seconds.
In this case, only keep-alive requests are sent. (Configuration messages are not “sorted”). The transaction number
is incremented only once in this case and a consecutive number in the data range of the CAN message is increased
for each new attempt to send. The TAN may no longer be incremented in this case (error). The last received
response is still in the input process image. If the TAN was to be incremented now, the current message would be
declared as valid upon reaching the last valid TAN. (A received TAN would then be the same as that which was
sent).
If the PICO controller again responds to keep-alive requests with keep-alive responses with the same TAN, the PDA-
available flag will be set and configuration messages can once again be “sorted”. The indicator for higher protocol
levels (whether or not configuration data can be sent) is the PDA-available flag.
It is possible to specify that a configuration data message (request) should be sent to the PICO controller via the
input/output trigger configuration data (Trigger_CD). In this case, the data is copied into the process image and the
timeout timer is simultaneously started.
If the PICO controller responds to this request with a response having the same TAN, the received data can be
analyzed. To analyze the response data, a check is first performed to determine whether or not the code value is
1, 2, 6, 8, 10, 11, 15, 21, or 22. If this is the case, the received data is interpreted as REAL. If the code has another
value, the data is interpreted as INT. Upon sending the values, the same principle is used to decide which value (INT
value/REAL value) will be copied into the process image.
PICO Controller 2+2-XCH-V3
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