Hello All

I need some help with SLC Message instruction.

The question I would like to ask is:

In the program i have a MSG intruction that is using Channel 0 DF1 to message to another PLC on channel 0 this is using a SLC 5/04 CPU.

I have now upgraded to a 5/05 CPU and have have now set the message instruction up for Channel 1 (Ethernet) but i have noticed that the control block length went from 14 to 51, is this normal? and is the 1st word of the control block which contains the block layout (error messages and status bits) still the same? Will this instruction work the same as the error bits of the message instruction (bit 12) are used through out the program.

Please Help Time is of the essence

I am relatively sure your DN, ER and TO bits are in the same location. Check the help file for RSLogix to be sure. It lays out the 14 and 51 length blocks in detail. And yes when going from DF1 of 14 length which has only a limited addressing scheme, to Ethernet with a length of 51 the extra space holds the Target IP, and Gateway information.

I was under the impression that MSG was only available in CompactLogix and ControlLogix (RSLogix 5000 s/w), and that an EEM (Ethernet Explicit Message) was the thing to use with SLC 5/05. My information, though, is secondhand....as it was told to me by a customer we (Opto 22) has that is (successfully) communicating from his SLC 5/05 to Opto 22 SNAP I/O.

I concur with Bob the extra words are for the target info.

I just checked my RSlogix 500 V6.3 and EEM is not a recognized Instruction for that version you must use MSG. Now perhaps somebody running the newer versions of RS500 can check the instruction help about EEM.

i have check my RSlogix v8, EEM is a instruction in SLC 5

"Message Instructions" are the framework for quite a few different data exchange protocols in the Allen-Bradley world.

In the ControlLogix, an MSG instruction can perform a ControlLogix tag read or write, or an SLC/PLC data table read or write, or a block transfer to an I/O module, or a CIP Generic message to a CIP object inside the controller, a module, or a network device.

In the SLC family, MSG instructions perform SLC/PLC data table reads and writes.

SLC-5/0x controller MSG instructions that use the "legacy" protocols DF1, DH-485, and DH+ have a control block that uses 14 Words of data.

SLC-5/05 controller MSG instructions that use Ethernet have a larger control block of 51 Words.


In the SLC-5/03, /04, /05 controllers Firmware Series C FRN 10, three new instructions were added for CIP network communications: DEM, CEM, and EEM.

DEM performs a DeviceNet Explicit Message with the 1747-SDN module. It replaces the old "copy an array into an M-file buffer" mechanism.

CEM performs a ControlNet Explicit Message with the 1747-SCNR module. Similarly, it replaces the old buffer mechanism.

EEM performs an EtherNet/IP Explicit Message with the SLC-5/05 controller's Ethernet port. This is a completely new feature for the SLC-5/05.

Because the EEM allows access to CIP Objects, it includes the ability to read and write Assembly Objects, which are used by most I/O devices, including the Opto-22 EtherNet/IP brain.

The EEM does not transform the SLC-5/05 into an I/O scanner; it does not configure, create, or run an I/O connection. But it does allow access to the underlying Assembly Objects that the I/O connection would use with a ControlLogix.

Because the I/O connection watchdog can be ignored by an Opto-22 Ethernet/IP brain, this makes data from those devices particulary well suited to access by an SLC-5/05 and the EEM instruction.

Ken - excellent info. Thanks.