Insights in Controls and Automation

Ethernet IP ports, Ethernet Rings on new Allen-Bradley PLC CPUs.

  

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Two Ethernet ports on Allen-Bradley PLC CPUs.  Many of Rockwell’s new products are coming with TWO Ethernet ports.  This has several advantages to users: 

1. Easy layout for an Ethernet “Ring”.  Multiple PLCs can be “daisy-chained” to give a Ring layout.  If the Ethernet cable is severed, theoretically there is a second path to allow communications.  While not a fully redundant network, it is an inexpensive way to get most of the way there. 
2. Ethernet IP for motion control.  Rockwell is now promoting the use of their Ethernet protocol for motion control.  This means that the second Ethernet port can be available for communications directly to a VFD if desired. 
3. Separtate switch is not needed. For simple systems such as one PLC and one HMI, the second port eliminates the need for a separate switch.  Usually you would add a switch to this two node system so you can plug a laptop or other device in for troubleshooting, or to connect to some larger network. That is included in the unit.

There is of course a trade-off.  With this arrangement you have one expensive CPU instead of an expensive CPU and an inexpensive switch.  If the separate inexpensive switch fails, you can replace only that.  If the switch that is internal to the CPU fails, then you need to replace the CPU.  When you decide on eliminating a separate switch, you must balance this concern against the advantages listed above.