CAN Bus

Inhibit Time for EMS CAN Repeaters

EMS CAN repeaters 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:

Baudrate
Bit time
Recommended inhibit time
Product #
Name postfix
500 kB/s
2 µs
500 ns
12-xx-x02-xx
-5
250 kB/s
4 µs
500 ns
12-xx-x02-xx
-5
125 kB/s
8 µs
1000 ns
12-xx-x03-xx
-10
50 kB/s
20 µs
2000 ns
12-xx-x04-xx
-20
20 kB/s
50 µs
5000 ns
12-xx-x05-xx
-50
10 kB/s
100 µs
10000 ns
12-xx-x06-xx
-100