In this Schneider Electric white paper: Reduce Ethernet Connected Cost with Daisy Chain

The author provides a daisy chain Ethernet topology in Figure 3:

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?


The 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.

  • The "tee" switches are very simple devices. They don't participate in STP, so you can chain up to 32 (per their specs.) I've chained over 40 Sun v20z's without issue. (their management engine is a 3-port single-chip switch) – Ricky Feb 18 at 11:13
  • So. they rely on a single STP-capable switch that thinks it's linking with itself? Gosh, that's a horrible setup... – Zac67 Feb 18 at 11:16
  • Pretty much. On paper, it certainly does sound like a recipe for disaster, but in practice, it works just fine. I wouldn't try to run the server's main ethernet off that string, but for OOB management, it works fine. In the OP's case, for a collection of PLC's, it should work just fine as well. – Ricky Feb 19 at 2:42

To quote your referenced document:

As shown in Figure 2, a daisy chain device has 2 embedded Ethernet ports which function as an Ethernet switch, as well as an interface to the local device. This allows information to flow to the device, or flow through the ports to other devices in the daisy chain.

Ethernet Daisy Chain allows devices to be cabled together in series using standard Ethernet cable, similar to legacy fieldbus’ without the need for additional Ethernet switches. Cabling devices in series permits flexibility locating devices, eases installation, and lowers infrastructure costs.

The entire point is to use ethernet. (otherwise, you're back to the old fieldbus/modbus/rs-485 arangment.)

[Note: the claim of reducing ethernet switches is a bit of a lie. Each of these "tee"s is a 3 port switch, just one built into the device.]

  • Yes technically increasing the number of switches, but reducing the amount of wiring the number of boxes and the number of power supplies. Embedded 3 port switch chips are cheap and the switch chip replaces the PHY chip so it often doesn't even represent an increase in the number of chips. – Peter Green Feb 23 at 15:44

the magic here is your end hosts (nodes that you control with your plc). Each should have two ethernet ports allowing you to go hop-by-hop. See the dotted notation on the right. That is actually a connection that is blocked by the ring algorithm so that a full loop does not occur (standby link). If you lose connection anywhere, the system will detect it and enable the ---- standby link. If you ditch the switch, you can connect your PLC directly to the first connection point to the left side, but beware that this becomes a chain (you don't have the ---- standby link). if you lose connection, there is no redundancy.

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