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CAN Bus Communications Decoded: Transmission Speed

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Transmission Speed

02.17

00:00 - At this point, we've determined how we're going to wire the bus connecting all our CAN devices to one another and how those devices are going to agree on what voltage levels on the bus actually mean.
00:10 Now we need to put some sort of structure around how we're going to actually transfer useful data.
00:16 This structure begins with the transmission speed.
00:19 As mentioned, CAN is a digital network protocol transmitting 0s and 1s.
00:23 Every device on the bus needs to agree how quickly these 0s and 1s are going to be sent and received or it will be possible for a fast transmitting device to send and receive several 0s and 1s while a slow receiving device isn't looking.
00:38 The slow device will miss most of the data and end up with a garbled incomplete message.
00:42 There are 3 common speeds used in the performance automotive world and these are expressed in terms of bits per second.
00:49 These are 250,000, 500,000 and 1 million bits per second.
00:54 More commonly, they're referred to as 250 Kb, 500 Kb and 1 Mb per second.
01:00 Less commonly used but still occasionally seen as a slower 125 Kb per second speed.
01:06 Most performance automotive CAN networks will run at 1 Mb per second as this speed has been proven to be reliable as long as we've followed our bus wiring rules.
01:15 This faster speed allows us to transmit more data within a smaller time period which is always advantageous.
01:21 Particularly when we're logging variables that change quickly.
01:24 However, safety critical systems, particularly those in OEM vehicles will often transmit at slower speeds.
01:30 This is because as we reduce the speed of communication, we increase the noise immunity and reliability of the network.
01:36 Reducing the communication speed also reduces the speed at which the voltage levels on our CAN high and CAN low wires change, therefore reducing the electrical noise that is being radiated out from our bus wiring.
01:49 I'm yet to strike a situation where switching to a transmission speed slower than 1 Mb per second has solved a data transmission problem but I have absolutely encountered these slower speeds when interfacing to OEM applications so it's important to know that they are out there.