EMS CAN Bus Repeaters and CAN Bus Fiber Optic Transceivers are offered with a parameter called inhibit time (sometimes referred to as blocking time or hold 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.
For CAN FD-capable units, the relevant baud rate is the nominal CAN baud rate used during the arbitration phase, not the higher data-phase baud rate.
Different product variants are available with different preset inhibit times. The table below provides recommended inhibit time values for common CAN baud rates and identifies the corresponding product variant.
| Baudrate (Nominal for CAN FD) | Bit time | Recommended Inhibit Time | Product Number | Product Variant |
|---|---|---|---|---|
| 500 kB/s | 2 µs | 500 ns | WU-12-xx-xx2-xx | -5 |
| 250 kB/s | 4 µs | 500 ns | WU-12-xx-xx2-xx | -5 |
| 125 kB/s | 8 µs | 1000 ns | WU-12-xx-xx3-xx | -10 |
| 50 kB/s | 20 µs | 2000 ns | WU-12-xx-xx4-xx | -20 |
| 20 kB/s | 50 µs | 5000 ns | WU-12-xx-xx5-xx | -50 |
| 10 kB/s | 100 µs | 10000 ns | WU-12-xx-xx6-xx | -100 |
Technical Background
When a CAN signal changes from recessive to dominant on one side of the repeater, the transition is propagated to the opposite side, where the signal also becomes dominant.
When the original signal returns to recessive, the remote bus segment may still appear dominant for a short period due to propagation delays and bus capacitance. At that moment, the repeater cannot determine whether the dominant level on the remote segment is simply the previously propagated signal that has not yet decayed, or a new dominant signal generated by a node on that segment that should be propagated. This situation can occur, for example, during the arbitration phase of the CAN protocol. To prevent retransmission of the same dominant bit, the repeater temporarily suppresses reverse signal propagation. This suppression interval is referred to as the inhibit time.
The inhibit time must be long enough to prevent false retransmissions, but short enough to avoid interfering with normal CAN communication. For this reason, selecting an inhibit time of approximately 10–20% of the bit time is recommended.
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