RSLogix MSG

Good afternoon all,
I'm not all that familiar with the Allen Bradley logic. I have two PLC's that are networked together via DH+.
The Master PLC has two MSG statements (two rungs) of the type "Peer-to-Peer".
One is Write and the next one is Read.
The Write Target is 500CPU, Local, Control Block is N9:0 and the length is 14.
The Read Target is 500CPU, Local, Control Block is N9:15 and the length is 14.
They are both executed by one timer on two separate rungs.
The Write is Timer N.O. and the Read is Timer N.C.

My question is, what is N9 and what is the length of 14?

The slave does not have any references to N9:0 or N9:15.
I can paste screen images into an email if needed.

I appreciate any help I can get.
Regards
Ray

The MSG instruction in an SLC-500 controller needs a "Control Block" for each MSG instruction. For messages on DF1 and DH-485 networks, this block is 14 words long, and for messages on Ethernet it's 51 words long.

The Control Block holds the status of the MSG instruction, as well as the rest of the stuff you configure inside the MSG instruction configuration window; source and destination data tables, node numbers, etc.

Make sure your MSG instruction control blocks don't overlap; the ones you mentioned do not.

The control block has no relation to the target controller's data table.

Thank you! From what I see in the program, individual bits are turning on individual bits in N7:10, N7:11 and N7:12. Those are then being read in the same N channels in the slave PLC where they perform output functions in the slave. The slave has a couple of MOV statements moving input channesl 1 and 2 to N7:15 and N7:16, respectively.
Since the setup in the master PLC for the MSG has 3 for the "Size in Elements", am I correct in assuming that the slave PLC can read N7:10 thru N7:12? (which have been addressed in the Master PLC)
Regards
Ray Flores