Support Note Bit rates, sample points and cable length. Question: What is the maximum cable length at a certain bit rate and please give me some hints where to put the sample point?! Answer: We recommend that you use the table shown below. These settings are taken from the CiA CANopen Specification 3.01 and we think they are great. Remark, please read the notes under the table. Bit rate Bus length Nominal bit time Number of time quanta per bit length of time quantum Location of sample point 1Mbit/s 25m 1 micro sec 8 time quanta 125 nano sec 6 time quanta (750 nano sec) 800kbit/s 50m 1,25 micro sec 10 time quanta 125 nano sec 8 time quanta (1 micro sec) 500kbit/s 100m 2 micro sec 16 time quanta 125 nano sec 14 time quanta (1,75 micro sec) 250kbit/s 250m 4 micro sec 16 time quanta 250 nano sec 14 time quanta (3,5 micro sec) 125kbit/s 500m 8 micro sec 16 time quanta 500 nano sec 14 time quanta (7 micro sec) 50kbit/s 1000m 20 micro sec 16 time quanta 1,25 micro sec 14 time quanta (17,5 micro sec) 20kbit/s 2500m 50 micro sec 16 time quanta 3,125 micro sec 14 time quanta (43,75 micro sec) 10kbit/s 5000m 100 micro sec 16 time quanta 6,25 micro sec 14 time quanta (87,5 micro sec) The table entries are an example based on the follow acceptance: 1. Oscillator frequency 16 MHz +/-0.1% (1000 ppm) 2. Sampling mode Single sampling SAM = 0 3. Synchronization mode Recessive to dominant edges only SYNC = 0 4. Synchronization jump width 1 * tq SJW = 0 5. Phase Segment 2 2 * tq TSEG2 = 1 Note 1: Rounded bus length estimation (worst case) on basis 5 ns/m propagation delay and a total effective device internal in-out delay as follows: 1. 1M - 800 kbit/s: 210 ns 2. 500 - 250 kbit/s: 300 ns (includes 2 * 40 ns for optocouplers) 3. 125 kbit/s: 450 ns (includes 2 * 100 ns for optocouplers) 4. 50 - 10 kbit/s: 1,5 tq; Effective delay = delay recessive to dominant plus 5. dominant to recessive divided by two. Note 2: For bus length greater than about 200 m the use of optocouplers is recommended. If optocouplers are placed between CAN controller and transceiver this affects the maximum bus length depending upon the propagation delay of the optocouplers i.e. -4m per 10 ns propagation delay of employed optocoupler type. Note 3: For bus length greater than about 1 km bridge or repeater devices may be needed. If you need more information to understand this subject, follow this link and read the KVASER CAN school online! Contact our Support Team. Back to Support Notes Index >>