We are designing a surge suppressor for a device net application. I need to know the typical voltages levels that are seen in the data lines CAN-L and the CAN-H terminals. Please indicate which of the following will apply 1) Is the CAN-L at -5 volts and the CAN-H at +V ( differential mode) 2) Is the CAN-L reference and the CAN-H at 5 volts with respect to CAN-L common mode) 3) I am assuming the data level is 5 volts is this correct Thank you for your help. the reason we need to know this is to select the right voltage protection level and the appropriate protection mode. Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net . A254) DeviceNet is a differential communications system. The CAN signal lines have two states, Dominant and Recessive. Further, CAN_L and CAN_H lines are biased at a nominal 2.5V in the recessive state with respect to the local V-. In the dominant state the lines are driven to a differential level of 2 volts. This means the CAN_H line can be as high as 3.5 Volts and the CAN_L line will be as low as 1.5 Volts. Because the voltage levels at a given transmitter are referenced to the local V-, Voltages seen at far ends of the network can appear at higher or lower with respect to the local V-. The offset in V- is a function of the DCR of the network and current in the V- of the network. This can cause CAN_H and CAN_L^s common mode voltage to rise by roughly 4.65 Volts or drop by 4.65 Volts. For example, if the current through V- caused a 1.5V drop across V- (end to end), then the CAN_H and CAN_L voltages could be 1 Volt recessive. In the dominant state, 0 Volts for CAN_L and +2 volt for CAN_H. There are small currents in produced by the transceivers bias networks that have some effect on reducing the amount of offset in CAN_H and CAN_L. One thing to remember in adding surge devices to the CAN bus, is that the system is very sensitive to capacitance. A node^s differential capacitance must be no greater than 25pF. The transceiver is approximately 11pF. The capacitance specification has to include stray capacitance in the traces, internal cable and connector capacitance. Some manufacturers already include varying degree of protection devices across CAN_L and CAN_H that add to the 25pF capacitance. Also in a shielded system most of the surge currents travel down the shield. DeviceNet has an optional shield termination circuit, which consists of a parallel R/C circuit (1M Ohm and .01uF.) Our experience has shown that the capacitor needs to be protected during surge testing. Some designers may use a transient device to protect the capacitor from damage. When surge testing the unshielded system, all four lines (V+, V-, CAN_L and CAN_H) are surged at the same time (common mode). In this test configuration, a protective circuit is required to ensure that the CAN_H/L lines never exceed the transceiver^s allowable common mode and differential mode voltages.