CAN Repeater Inhibit Time

EMS CAN Bus Repeaters and CAN Bus Fiber Optic Transceivers are offered with a parameter called inhibit time. It is important to choose the correct value for this parameter that the repeater will work as expected in a dedicated system. It is recommended that an inhibit time of 10-20% of the bit time be used. The background is explained in the following.

If the CAN signal goes from recessive to dominant on one side of the repeater it is propagated to the other side and here the signal also changes to dominant. If the first signal goes back to recessive, there is still a dominant level on the other side, it is held dominant by the capacitance of the cable. In this moment the repeater cannot decide, if this dominant signal on the second side is the old signal and will fade out or if it is a new signal originated by a node on the second side and should be propagated. This effect can occur e.g. during the arbitration phase of the CAN protocol. The consequence is that at this moment for a short period of time a back transmission of the dominant signal has to be omitted.

The following table should give an overview about commonly used values:

BaudrateBit timeRecommended inhibit timeProduct #Name postfix
500 kB/s2 µs500 ns12-xx-xx2-xx-5
250 kB/s4 µs500 ns12-xx-xx2-xx-5
125 kB/s8 µs1000 ns12-xx-xx3-xx-10
50 kB/s20 µs2000 ns12-xx-xx4-xx-20
20 kB/s50 µs5000 ns12-xx-xx5-xx-50
10 kB/s100 µs10000 ns12-xx-xx6-xx-100

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