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The Reality of Serial Bus Speeds

Serial buses and networks such as Ethernet, CAN, USB, and Firewire are popular.   But you can’t just say “I need to transfer 1Mbps, therefore I can use CAN.”  You need to understand a bit about the physical layer and your chosen software layers before you can pick a bus that will work.

All serial buses have some protocol overhead, and software layers add even more.

I’ll take a quick look at CAN:

  • CAN messages can carry between 8 and 64 bits of data.
  • At 1Mbps, the maximum actual throughput is roughly 17543 messages/sec (1 byte data) to 8771 messages/sec (8 byte data) or 140,344 to 561344 bits per second of actual data.
  • CAN does have many good features, like extremely fast arbitration (much faster and more predictable than CMSA/CD Ethernet), and producer/consumer messaging.
  • So CAN is a good choice for a real time network that isn’t transferring a lot of data.  It’s even better if many nodes need to consumer data produced by one node (so the data is only sent once, unlike in a master/slave network where the master has to receive the data and then send it out again to each slave node).
  • On the other hand, it’s not a good fit for a 1M sample/sec 16-bit ADC; high speed USB 2.0 or 100Mbps/1000Mbps Ethernet would be better choices.
  • There’s also a cable length / speed trade off; as the bit rate increases, the maximum cable length (and branch length) decreases.
  • The maximum number of messages gives an idea of what kind of update rates you can realistically see.  For example, a 100 node CAN network could handle a maximum of roughly 175 1-bytes messages or 88 8-byte messages per node per second.  So a 1 msec update rate for all nodes is impossible, but a 10 msec rate might be achievable.

Ethernet has its own set of considerations.  Just a few:

  • Ethernet has significant protocol overhead, especially for small data sizes.  (Summation frame systems such as EtherCAT reduce this, but require non-standard hardware).
  • If you’re using hubs, you have to take collisions into account.
  • If you’re using switches, you have to consider the time the switches add.
  • Ethernet and its typical protocols require a lot of resources; if your product uses an underpowered controller, even if it’s physically on 100BaseT, it might only be able to manage 1Mbps or less.
  • For long distances, the time traveling between nodes can be significant.
  • Higher level protocols such as TCP, ftp, NTFS, etc can add substantial delays and additional overhead along with their added features.

I’m not a USB expert, but I know USB 2.0 can’t deliver actual data at its advertised speed.

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