Refer to the Miswiring Protection circuit in Figure 9.4 (Volume 1, Chapter 9, Release 2.0). A rough estimate of the current required for the transceiver is 100mA - 70mA for the Philips PCA82C250 (in dominant mode) and 25mA for optos. If V+ is 24 volts, this means the regulator power is (24 - 5)V * 0.1A = 2 watts. This is a problem. I don^t have room big power parts. Can a lower value of V+ be used. What about 12 volts? Answered by Matt Kuzel, Physical Layer SIG Chairperson,
e-mail: kuzel@voyager.net
A120) These current and power levels are peak levels. Average current (which is much lower) should be used for establishing power capability of components. Many designers only consider recessive current on the transceiver, since it is so rarely dominant. A possible worst case might be the device continuously re-trying a message because it is the only one on the network. An example of this would be simply powering the device up by itself. The device will continuously retry the duplicate MAC ID check request message. One way to handle the power with smaller parts is to divide the power by using a pre-regulator or voltage follower before the 5 volt regulator.