Run RS-485 two wire cable with analog signal in one multicore cable

Hi,

Can I have genius thoughts on if there will be any concern if I run RS-485 2-wire cable with Analog input in one multicore cable lets say twisted pair multicore cable for distance of 500 mtr.

and recommendation for a distance near 1000 mtr.

Thanks
Faseelath

I doubt the digital signal will affect the analog signals in a multicore cable; RS-485 is such low voltage, negligible current.

The cable capacitance will affect the signal. You should be looking at cable rated for communication - it has lower capacitance.

Although it's called RS-485 2 wire, it really needs a 3rd wire for signal ground. Hopefully the RS-485 is isolated because at those distances the probability of a ground loop is fairly high.

Distance affects the useable baud rate, as the chart of theoretical baud rates vs distance shows.
Note that 500m = 1600 feet; 1000m = 3300 feet (3.3k ft):

I've done it and it's worked fine. Actually used a an 8-pair cable once - two pair for analog, one pair for rs-485 and two pair for ethernet with one spare pair. I set up the ethernet device for 10M and it worked fine. Still works, as a matter of fact.

1000 m is pretty far to take a chance unless the cable is already ran and trying wont hurt. Otherwise, I would run the proper cable so you dont have to come back and redo it all.

Thanks guys.

I suggested my client same to run special cable for long distance like 1000m.

Please do check the voltage drop for signals like these to. Depending on your analog signal, the voltage drop could be significant.

Have you considered putting in a remote station and run fiber optics or something similar?

Voltage drop is generally not an issue when
- distance limits are observed
- number of multidrop nodes is held under the limit
- field added biasing resistance is not too low, which would increase bias current and drop supply voltage by driving 'excess' bias current.

The EIA-422 spec has cable recommendations: 24 AWG twisted pair cable with a shunt capacitance of 16 pF per foot and 100 ohm characteristic impedance. These recommendations are generally considered as suitable for RS-485 was well, although 120 ohm characteristic impedance is probably more common.