Special configuration features

Certain special features need to be noted for configuration purposes.

Arrays

Several data points can share a common peripheral address. In this case the individual elements are differentiated using a subindex. The subindices need not be consecutive. For example a data word can be interpreted as an array of 16 bits. The peripheral address then codes the hardware address for this data word, and the subindex defines the position of the bits within the word. One should note, however, that the same conversion type must be specified at the driver for all the elements in an array.

bit string

Bits cannot be set or read individually, since access to the PLC memory can only be on a word or byte basis. This is why all the bits belonging to a peripheral address are always read or written, rather than an individual bit. For configuration purposes this means that such a data point cannot be set as an output with single registration.

Spontaneous transmission from the PLC

The driver supports spontaneous transmission from the PLC, which means it will receive data that does not come in response to a FETCH command, but in a spontaneous SEND command from the PLC. The target address in this message can be set as required. Since the driver can only identify the data point using this address, however, the address field of the message should contain the source address of the data. In order to be able to identify the associated data point from the hardware address, this data point must have the corresponding peripheral address and the subindex '0'.

Polling the PLC

Inputs can also be read periodically. In this case one should note that communication over a serial interface is rather slow. A baud rate of 9600 baud means for example a data transfer rate of < 1 kB. The available useful data transfer rate is even lower. Thus the polling times should not be set too short. In order to avoid the internal buffer overflowing, the driver only polls a data point if there is no identical task waiting. This means that even with a polling time of 0,1 seconds, a data point is not necessarily polled 10 times per second. Single queries, on the other hand, are always executed irrespective of whether another identical query is already waiting.