New S7A Option: Digital signals with timestamp

Starting with version 7.40, build no. 105 the S7A driver now offers a new function which allows you to receive up to 4096 digital timestamped signals from the PLC and to forward these signals to the clients (iFIX and/or OPC).


By the “regular” polling of the PLC data area, the timestamp will be generated by the SCADA System (within the S7A driver). Due to communication delays between the PLC and the driver, such a timestamp does not represent the real time of the signal (the time when the signal actually occured) .


With the new S7A option “Digital Signals” it is now possible to generate the timestamp in the PLC at the time the signal state change occurs and to transfer this timestamp together with the related signal state to the SCADA system.
Furthermore, the signals and their timestamp will be buffered within a FIFO buffer of the PLC and transferred asynchronously to the S7A driver. The buffering ensures that no signal changes will be
lost (assumed that the size of the FIFO buffer is sufficient) in case of fast signal changes and/or low communication speed between the PLC and the S7A driver.
To transfer the signals and its timestamp to the S7A driver a so-called transfer buffer (data block) with a dynamic length is used. The S7A driver periodically polls a single handshake byte of this transfer buffer. The handshake byte is set by the PLC when new signal changes occurred and buffered in the FIFO. Afterwards the S7A driver just reads the amount of data bytes which are allocated in the transfer buffer to transfer these signal changes and their timestamps.
Due to this transfer method the communication traffic between the PLC and the S7A driver is reduced to a minimum and even a large number of signals can be transferred effectively by a relatively small transfer data block.
The following diagram shows an overview of the driver’s new functionality.

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