This of course can all be calculated out at nominal wire temperatures and conditions. CAT5 cables use 24 awg wire size. Two webpages I consulted (here and here) specify that the resistance of 24awg wire is equal to 0.0842 ohms/meter.

With a CAT5 cable run of 20 meters the total length of wire is twice that for down and back so 40 meters. Wiring three conductors in parallel reduces the resistance to one third. So total power wire resistance is:

Total Resistance = (40 * 0.0842)/3
Total Resistance = 1.12 ohms

Simple application of ohms law says that if you were dropping from 12V to 7V down the wire (a drop of 5 volts) then the current being drawn was:

Current Draw = 5V / 1.12 ohms
Current Draw = 4.46 Amps.

At less than 300mA per clock you must have been wiring to over over 12 clocks on the down stream cable.

One should consider some important factors when delivering power down the cable in RS422 / RS485 applications over the same cable. Receivers for these types of interfaces have a certain common mode voltage range. This is the voltage range over which both signal wires carry a common voltage level and the receiver can still operate safely and be able to detect differential signaling on the two wires. Take for example the LTC1485 part from Linear Technology. It can work properly in the presence of a common mode voltage drop of -7 to +12 volts.

Delivering power over the wires to a clock means that the ground level at the destination clock gets elevated to above that level at the source end. In the case you stated with a 5V total supply drop the GND elevation is 2.5V. This makes a RS422 receiver at the destination see signalling at a nominal common mode of -2.5V. If you had signalling protocol in the other direction back to the source the receiver there would see signalling with a nomimal +2.5V common mode voltage. (I say nominal because the current drawn over the signal wires and through the terminations on either end will add or subtract from the bulk common mode voltage that is produced by the power delivery over the cable.

For cable powered remote devices like this there can be an advantage of wiring to all the remote devices in a star configuration instead of as a serial daisy chain configuration.

POE has the potential to be more robust than RS422/RS485 for remote wired power applications because Ethernet communications links are transformer coupled and thus are much more immune to common mode voltage drops between the endpoints. For sure the use of 48V feeds at lower current has the potential to reduce common mode drops by a factor of four over a 12V system (at the same delivered power levels) but it is the transformer coupling that helps the most. DC coupled RS422/RS485 is often chosen for an interface because of its apparent simplicity and ready application to support communications via simple asynchronous UARTs.


Michael Karas