• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Measuring resistance on an RS-485 line

Status
Not open for further replies.

bh00

New Member
Hi,

I'm looking for some sort of component or "method" for measuring to see if there is termination in place on an RS-485 (DMX) line. It needs to be placed at the same end of the circuit as the line driver, and ideally not present any load or other problems on the line. Some sort of logic output to a microcontroller to basically detect if the 120:eek:hm: termination resistor is present across the lines at the other end is what I'm after. How could this be done?

Cheers
 

Leftyretro

New Member
bh00 said:
Hi,

I'm looking for some sort of component or "method" for measuring to see if there is termination in place on an RS-485 (DMX) line. It needs to be placed at the same end of the circuit as the line driver, and ideally not present any load or other problems on the line. Some sort of logic output to a microcontroller to basically detect if the 120:eek:hm: termination resistor is present across the lines at the other end is what I'm after. How could this be done?

Cheers
How about placing a small value resistor is series with the loop and measure the voltage drop while the link is idle, that should give you an idea if the line is terminated or just 'seeing' the high impedance of the far end receiver. To automate if you would have to have one driver enabled but sending only a steady 'mark' condition.

Good luck
 

bh00

New Member
Hi, This sounds like a good way I might experiment with - I guess you mean create it with a sort of potential divider, so if the 120:eek:hm: resistor is present it will cause a voltage drop. Do you have a schematic of how this would be arranged? As DMX is a fast signal that needs to be constantly carrying data though I don't quite understand how it would fit in as there is no opportunity to check really. Ideally I want it to be continuously checking and alert as soon as the termination is added or removed.
 

Leftyretro

New Member
bh00 said:
Hi, This sounds like a good way I might experiment with - I guess you mean create it with a sort of potential divider, so if the 120:eek:hm: resistor is present it will cause a voltage drop. Do you have a schematic of how this would be arranged? As DMX is a fast signal that needs to be constantly carrying data though I don't quite understand how it would fit in as there is no opportunity to check really. Ideally I want it to be continuously checking and alert as soon as the termination is added or removed.
No schematic, just planting a seed idea. Not a potential divider, just a series resistor, measure the voltage drop across a series resistor will tell you the total current flowing on that conductor. A wire in a terminated loop will draw more current then a unterminated wire, no? You would be suprized how many solution involve just applying ohms law ;)

Good luck
 

bh00

New Member
And to effectively measure the voltage drop across a series resistor this way I'm guessing if I used two ADC's on a PIC I can measure each side and work out the difference? Or is there a simpler way?
 

Sceadwian

Banned
If one side is ground you don't need to measure both ends, just the 'high' side. I don't know about PICs but the AVR's I work with usually have a differential input (that works within the supply range) at 8 bits resolution on their ADCs.
 

mneary

New Member
The DMX signal is kind of fast (4 microseconds per bit) but the idle time (usually a few milliseconds) and the break time (88 microseconds or more) are possible moments you can take the measurement. This gives you both signal polarities.

There might be another way: If the termination disappears, then the driver would see a reflection from the far end, in phase with the driver. The overshoot should be obvious. A peak detector looking for a difference between the peak and average voltage should be able to catch an unterminated line.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top