Data conversions: PLC-5 polled mode

All polling mode data reads are done with a type of Data Highway Plus command that specifies only a file number and an element, but does not specify a data type. When the PLC-5 receives this command, the processor does not have a file type specified; it cannot check to see if the file type matches the file number.
When the processor builds a response packet containing the requested data, it simply goes to the start of that area of physical memory (the processor knows how to translate a file number/element into a physical location in its memory), and starts reading the bytes in sequential order out of its memory and into the packet. When RSLinx Classic receives the packet from the PLC-5, it must convert the raw data into a format suitable for transmission to the Windows application.
RSLinx Classic performs the conversion by looking at the file type specified by the address that the Windows application requested. If the address was
T4:0.ACC
, then DDE assumes that the bytes correspond to integers and translates the bytes accordingly. If the address was
A4:0
, then RSLinx Classic assumes that the data is ASCII. All data types in the PLC-5 are treated in this manner.
Therefore, it is possible for the Windows application to request the same data from the PLC-5 in a variety of different formats. It is perfectly acceptable (as far as DDE is concerned) to request
A4:0
and to read that data, although within the PLC-5 the file contains timers.
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