In this Schneider Electric white paper: Reduce Ethernet Connected Cost with Daisy Chain The author provides a daisy chain Ethernet topology in Figure 3:
To connect the loop to another device (I'm calling this a "tee"), the figure shows an ethernet switch. Is there some device or method to connect to the PLC, without using an ethernet switch?
86.4k 4 4 gold badges 71 71 silver badges 137 137 bronze badges asked Feb 18, 2021 at 6:43 Intrastellar Explorer Intrastellar Explorer 133 1 1 silver badge 6 6 bronze badgesThe example you quote is for a specific application, where the data workflow is along the chain. Chains are considered a bad design for Ethernet in general - Ethernet works better in a tree topology.
There, you'd use two central switches and connect each device to both. In normal operation, the spare switch would use RSTP/MSTP to block all redundant connections, breaking the bridge loops.
is there some device or method to connect to the PLC, without using an ethernet switch?
Since you're basically connecting three ports, no, there's no alternative to a switch [1] . Note that with a ring topology, that switch needs to use RSTP/MSTP to avoid a bridge loop (or an alternative mechanism) [2] .
Also, the root switch has to be selected carefully, so that in normal operation the STP blocked link is where you want it. Note that the spanning tree depth/chain half-length may not exceed the RSTP design limits (7 with default settings). Using dumb switches [2] , you can build much longer chains, until their forwarding delays sum up to a significant figure or until the chained links become throughput bottlenecks.
[1] Connecting a "tee" into the loop requires three ports. You could connect these ports into a multiple-access, half-duplex collision domain using a repeater hub - limited to 100 Mbit/s and long obsolete. Or you can use a buffering bridge - a switch - that enables full duplex and allows for faster speeds.
[2] Using dumb switches that completely ignore 802.1D (quite common for small switches), you'd only need a single STP-capable switch. The dumb switches would literally forward the STP BPDUs from the smarter switch. That switch would essentially think its two ring ports would be connected to each other directly and it would subsequently block one of the ports.
