Rockwell Automation 2711 PANELBUILDER SOFTWARE USER MANUAL - Manual (Page 376)

PLC Programming Considerations
Appendix A
A-12
When using a PLC-2 (2/05, 2/15, 2/16, 2/17), the system must contain a
1771-SN subscanner module. The PLC-2 processors do not have built-in
remote I/O capabilities, but the 1771-SN subscanner module can
communicate with remote devices such as PanelView. The subscanner
must be located in the same rack as the PLC-2, and communicate to the
PLC controller by means of block transfers. The 1771-SN looks like an
intelligent I/O module to the PLC controller; it monitors discrete inputs
and controls discrete outputs.
The 1771-SN requires one PLC scan to receive the output data from the
output image table and another PLC scan to send discrete input data to the
input image table. Any devices connected to the subscanner are scanned
asynchronous to the PLC scan. The PLC controller then scans the
subscanner module during the I/O portion of the PLC scan.
You can connect up to seven remote I/O racks to the 1771-SN. Set
switches 1 through 7 on Switch bank 1 of the 1771-SN module to
correspond with the assigned remote I/O racks.
Your ladder program instructions should include read and write block
transfer instructions and a file move instruction (see Figure A.8). The
block length of each block transfer instruction is 24 words (assuming
remote I/O rack numbers 1 and 2). It is recommended that you select the
default block length (00) for the module, which permits it to automatically
select the proper number of block transfer data words. Whether or not you
condition your block transfer rungs depends on your application
requirements. If you enable both the read and write block transfer
instructions in the same scan, the module performs bi-directional block
transfers starting with a read.
Using PLCĆ2s and a 1771ĆSN
Subscanner with PanelView