Why Devicenet Baud Rate Changes Are Needed

WHY DEVICENET BAUD RATE CHANGES ARE NEEDED • The length of wire connecting the DeviceNet devices has resistance and capacitance elements. When the wire gauge is larger then there is the resistance per foot of cable. • The capacitance of a cable is based upon cable design, wire size, shielding, etc. • The RC (resistance times capacitance) time constant of a piece of cable will affect the rise and fall time of a data bit. • To recognize a valid bit of data the devicenet receiver must be able to time the data bits to the midpoint mark of a bit. If the bit timing is dramatically off then the receiver will not recognize the data stream. The slower rise and fall times of the data edges will cause a receiver to see shorter length bits which will lead to unrecognizable data. Logic data state determined at mid point of data bit.

Logic data bit

Effect of RC time constant of cable on data bit.

When the RC time constant effect becomes too long the logic state of the bit will no longer be seen by the receiver. Too fix this lowering the baud rate of the communications will lengthen the data bit width allowing the data to switch again. BAUD RATE OF DEVICENET Baud rate is defined as bits per second. Therefore to find the length of one bit merely divide the baud rate into 60 seconds. 60 = 12 × 10 −6 or simply 12 microseconds. Example for 500kb is 3 500 × 10 This means the logic would switch at 6 microseconds. As can be seen in the following example it takes four times longer for one pulse to switch at 125Kb (switch point at 24 microseconds) than it does at 500Kb. Therefore it can tolerate more wire length which increases the rise time of the signal. It also means it takes longer for message to be transmitted as well.

switching point 500 Kb

250 Kb

125 Kb