Speed and length
Every CAN bit has to be long enough for a signal to reach the far end of the bus and come back, because arbitration and the acknowledgement both depend on it. So the bit rate sets the longest bus: about 100 m at 500 kbit/s, about 40 m at 1 Mbit/s. This board also slows its own edges to keep the bus quiet, which costs a little of every bit.
The round trip inside a bit
When two boards arbitrate, each has to see the other's dominant bit before it samples its own, even if they sit at opposite ends of the bus. The ACK slot is the same: the sender reads a bit the farthest board drove. So a signal has to go out, through a transceiver at each end, and come back within one bit, with time to settle before the sample point, about three quarters of the way through.
Cable costs about 5 ns a metre each way. The rest is the transceivers.
The rule of thumb
| Bit rate | Longest bus, about |
|---|---|
| 1 Mbit/s | 40 m |
| 500 kbit/s | 100 m |
| 250 kbit/s | 250 m |
| 125 kbit/s | 500 m |
| 50 kbit/s | 1 km |
These are the figures CAN designers work from, not guarantees; cable, stub lengths and the number of boards all move them.
What this board adds
The SN65HVD230 has three speeds for its edges, chosen by its Rs pin. On the TK109 that pin goes to GND through 10 kΩ, which puts it in slope control: each edge takes about 120 ns instead of about 50. Slower edges radiate less and ring less on long, unshielded cable, which is why it is fitted.
The cost is time. Round the loop, from CTX to the bus and back out of CRX, the chip takes up to 185 ns, so the two ends of a bus spend up to 370 ns of every bit before the cable has had any. At 500 kbit/s that is under a fifth of a bit. At 1 Mbit/s it is over a third, which is why 1 Mbit/s is the chip's rating, fine on a short bench bus, and not a figure to plan a long run on.
When it does not work
This board's 10 kΩ slope control stretches each edge to about 120 ns, and the two transceivers' round trip to as much as 370 ns of a 1000 ns bit. On anything but a short bus that leaves too little. Go back to 500 kbit/s, which is what the boards were tested at.
The slowest that carries your messages. A few sensor readings a second fit easily into 125 kbit/s, which reaches several hundred metres and shrugs off more noise. Faster buys nothing a slow bus cannot do, and costs length.
Only by changing the board: the chip's Rs pin goes to GND through a 10 kΩ resistor, and replacing it with a wire puts the chip in high-speed mode, with faster, noisier edges. For a short bus at 1 Mbit/s that is the trade. It is not needed at 500 kbit/s.
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