Block read sizes

To read more than one value at a time, use a block read command format by adding length and format specifiers (see Generic item format). For this to work, you must know the single block maximum size limit. This size limitation is determined by the maximum quantity of bytes that can be moved within a single Data Highway packet, and varies based on which device generated the packet. If you are using polled mode, RSLinx Classic generates the packet. If you are using unsolicited mode, the processor generates the packet.

The maximum size is based on the maximum number of bytes that can be moved within a single Data Highway packet, and, when a request for data is made,

all

the members of the data table element being retrieved. For example, if requesting

T4:0.ACC

, a request is actually made for

T4:0

(Control word),

T4:0.PRE

, and

T4:0.ACC

. Therefore, it requests the accumulated value, as well as the control and preset values.

When determining the maximum number of elements to be read within a single block, consider both the maximum quantity of bytes that can be moved and the number of words within a single element. These values vary with both the originator of the data movement command and the type of data. For both polled and unsolicited mode, RSLinx Classic refuses the data request if the block size entered into the address field exceeds the maximum mode value.