Ethernet Networking of Microcontrollers with Daisy Chain Topology

[slayer2211]

Hi guys,

I am new with networking. I know daisy chain configuration is used in some of industrial automation. My objective now is to set up a network that consists of multiple STM32F107 MCU device through ethernet, with daisy chain topology.



Since the STM32F107 MCU comes with 1x ethernet feature, therefore, I am trying figure out if it's possible to place a dual port RJ45 connector in order to establish a daisy chain configuration (MCU A connects to MCU B, MCU B to MCU C, MCU C to MCU D,... and so on).

This MCU also requires an external PHY chip to interface the MCU's Ethernet MAC with signal from RJ45. In this case, I am looking for the specific PHY chip that can take two RJ45 and be configured for daisy chain topology.

Thanks!

 

[bigdogguru]

Since the STM32F107 MCU comes with 1x ethernet feature, therefore, I am trying figure out if it's possible to place a dual port RJ45 connector in order to establish a daisy chain configuration (MCU A connects to MCU B, MCU B to MCU C, MCU C to MCU D,... and so on).

A network protocol with similar topology, backbone/node, already exists for just such a purpose, CAN 2.0B. Most ARM Cortex-Mx series microcontrollers offer at least one CAN 2.0B port, many offer two CAN 2.0B ports. supports up to 1Mb/s throughput, designed with harsh environments in mind, forms the basis for many fieldbus implementations. Is relatively inexpensive to configure and implement from both an economical and system resource standpoint. The STM32F107 microcontroller is equipped with two independent CAN2.0B ports and would be much more economical and practical. Much of the overhead of a CAN 2.0B implementation is handled in hardware, rather than in a software stack, reducing the required code sizes of the CAN 2.0B or associated fieldbus software protocol stacks and the final application implementation.

This MCU also requires an external PHY chip to interface the MCU's Ethernet MAC with signal from RJ45. In this case, I am looking for the specific PHY chip that can take two RJ45 and be configured for daisy chain topology.

The daisy chain topology you are describing more closely resembles that of Fiber Distributed Data Interface (FDDI) or Copper Distributed Data Interface (CDDI), rather than IEEE 802.3 Ethernet networks. Without going into too much detail, implementing an IEEE 802.3 Ethernet network on a similar topology would require the implementation of a hub/switch at each node, which would present it's own issues and limitations, not to mention significant costs and complexity of design.

There are also a number of reasons to avoid a daisy chain network topology, one of the primary reasons is, each node could represent a single point of failure for the entire network.

It would be much more prudent to simply utilize one or more Ethernet switches and run a single connection to each node, rather than attempt to force the IEEE 802.3 standards to fit a network topology for which it was neither designed nor intended.

Or utilize a simple backbone/node topology and implement a CAN 2.0B or associated fieldbus network.


BigDog

 

[slayer2211]

[Hi BigDog,

Thank you for your quick response. I know CAN 2.0B can be used in such purpose as it offers more economical approach, but it's just that I am looking for possibilities for daisy chain networking with Ethernet protocol.

The daisy chain topology you are describing more closely resembles that of Fiber Distributed Data Interface (FDDI) or Copper Distributed Data Interface (CDDI), rather than IEEE 802.3 Ethernet networks. Without going into too much detail, implementing an IEEE 802.3 Ethernet network on a similar topology would require the implementation of a hub/switch at each node, which would present it's own issues and limitations, not to mention significant costs and complexity of design.

It would be much more prudent to simply utilize one or more Ethernet switches and run a single connection to each node, rather than attempt to force the IEEE 802.3 standards to fit a network topology for which it was neither designed nor intended.

So I assume the IEEE 802.3 standard Ethernet protocol does not suit well with this topology? What about daisy chain with RS 485 standard? Apart from that, what is difference between bus and daisy chain topology?It seems similar to me as all node are connected in a ring where info would have to travel from one point to another.

Thanks.

 

[FvM]

what is difference between bus and daisy chain topology?

The term involves some ambiguity. A daisy chain of interface cables can hide different communication topologies:
- a single full-length bus (or "party line") line in case of RS-485 or CAN
- a combination of point-to-point connections

As a special case of the physical point-to-point connection, the data can be immediately repeated to the next device (and possibly modified), e.g. INTERBUS, EtherCAT.

For ethernet, you have the option to connect a switching PHY like LAN9313 and connect the network in a daisy chain manner.

 

[Dan Mills]

The usual approach is to integrate a three port switch into each node (One switch chip plus two PHYs and magnetics usually), job done.

Note however that this costs you fault tolerance compared to an actual bus architecture as a powered down note will break the line.

Note that ring does not work (normally) with standard ethernet, you need spanning tree capable switches for that so the switch can identify a loop and break it (Modern ethernet is actually point to point between PHYs, there is no collision detection, this is not old CSMA-CD thin net anymore).

Half duplex RS485 (Or CAN) has the upside that powering off a node does not inherently break the line, unlike the case with switches on each node.