I am using a board with wired communication on RS485. I am using SN65HVD72, it specifies cable length to be 2000m.

The board on other side which is in field, i.e 30m away has 4 wire connection, i.e Vcc, Gnd, A, B. (where A, B are RS485 signals).


1. I am planning to use four core cable.
2. My field board has 80mA current consumption. Vcc is +5V. This +5V goes to 3.3V regulator which is used for board finally.


Question:
1. Will +5V be able to show at filed board which is 30m away from centeral board. Also keeping in mind 80mA cuurent consumption.

2. Will RS485 will be done fine?

3. What type of four core cable I should select

 

the voltage drop depends on the size of the wire. the rs485 driver you said you were using is speced for up to 2000 meters, 30 should be no problem

 

The cable must be twisted pairs. A and B signals must be paired.

 

Feed it with 6 volts.

 

I have selected this 4 core cable 0.5mm^2.
http://edgecdn.lappgroup.com/filead..._doku/datenblaetter/unitronic/DB0034302EN.pdf

1. Since loop resistance is 38.9ohm/Km. & my requirement is 50m or 100m in loop so total resistance = 3.89ohm, so drop at 80mA is .32V. Will work fine according to me?

2. Other is I didn't find characteristic
impedance in datsheet for termination in RS485 connections.?

I have only master & slave device (2 devices on Rs485)
& at 9600bps. Do I need termination resistance or I can ignore

 

Is the cable twisted pairs?

You cannot ignore termination. Use 120Ω at both ends.