In all cases except this one you won’t do it because the readings are inaccurate, but since we’re not looking for a specific reading it works just to know the network is active. Plus it won’t hurt the circuit to do this.
The 120 ohm resistor is usually called the end of line resistors and they are usually in the devices that are of course at each end of the line. You are correct that some of the devices are on a spur off of the main line so you could have one of these devices disconnected and still measure your 60 ohms. We have a rolling mill at work which uses basically a can buss to communicate with all of the I/O and devices distributed throughout the machine and sometimes the connections on some of the devices get dirty and the machine starts acting up because of bad communication on the can buss. I usually hang an Ohm meter across the can bus then go around to each device and give it a tap and if the connection is dirty I can see the resistance change and clean that connection.
I was taught to not test resistance with power running through a circuit, does it not matter?
In all cases except this one you won’t do it because the readings are inaccurate, but since we’re not looking for a specific reading it works just to know the network is active. Plus it won’t hurt the circuit to do this.
The 120 ohm resistor is usually called the end of line resistors and they are usually in the devices that are of course at each end of the line. You are correct that some of the devices are on a spur off of the main line so you could have one of these devices disconnected and still measure your 60 ohms. We have a rolling mill at work which uses basically a can buss to communicate with all of the I/O and devices distributed throughout the machine and sometimes the connections on some of the devices get dirty and the machine starts acting up because of bad communication on the can buss. I usually hang an Ohm meter across the can bus then go around to each device and give it a tap and if the connection is dirty I can see the resistance change and clean that connection.
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