I would like to know the impedance of electrical cord and its suitability to be used as a 485 cable.
I am working on a product that has very minimal data rate requirement, 9600 baud is ok. However its not possible to ship expensive cable with the product since the actual length that the customer needs is not known. I want to check whether the twisted cord typically used as electrical power wire in households would work. The max end to end cable distance is about 600 metres.
I would like to know the impedance of electrical cord and its suitability to be used as a 485 cable.
I am working on a product that has very minimal data rate requirement, 9600 baud is ok. However its not possible to ship expensive cable with the product since the actual length that the customer needs is not known. I want to check whether the twisted cord typically used as electrical power wire in households would work. The max end to end cable distance is about 600 metres.
Thanks and Regards,
WonderfulIOT
The wire length for any data transmission is the capacity between the wires. The data signal is a square wave. The greater the wire capacity, the more the corners of the square wave are rounded off, untill there is no longer a detectable square wave.
The maximum length for RS-232 at 9600 BPS is 50 ft. Over the years, other standards were created because the driver chips were improved. They are able to actually provide the power necessary to discharge the current stored in the data cable and reverse the charge and keep the original square wave. RS-485 was one of the attempts, so the supported cable length is increased for the same data rate.
If you go to RS-422, the drivers have again increased the charge/discharge current so the distance can be even further.
So, the use of any cable with twisted or not twisted wires needs to look at the capacitance of the wires per feet or meter or whatever length you choose, and pick the one with the lowest value. Such a parameter is seldom given today because with today's technology, such wiring is seldom done. You may have to actually measure the capacitance.
Paul
It's not just about capacitance, there's also the inductance a well as (series and parallel) resistance of the wires. At very high frequencies those factors combined result in a characteristic impedance, which is a constant factor, independent of the cable length. See also Characteristic impedance - Wikipedia .
The cable must on both ends be terminated with a resistance equal to the characteristic impedance, otherwise energy will be reflected back into the cable (!), standing waves will occur, and energy will be lost.
Coax antenna cables usually have 75 or 90 Ω, flat cable 300. In an another topic on this forum a twisted pair of wires (like telephone wire) was mentioned as 120 Ω~~, but I can't seem to find it now~~. I guess the "twisted cord" that OP wants to use will have an impedance of about 110 .. 130 Ω as well.
It would be extremely foolish to rely on my opinion or anyone else's opinion when you can easily buy a spool of the wire in question an actually try it yourself.
Paul