Cable Line Length

What cable line length should be designed depending on the distance to the interface converter location?

The limit length for each measuring line of network is 1 km, but note that in this case data will be transferred at minimum rates. At the same time, it must be taken into consideration that total resistance of the data line loop must not exceed 100 Ohm, and power supply circuit resistance must provide sufficient voltage (more than 9V) for the module at the remote (in relation to the interface converter) end of the measuring line. To construct measuring lines, it is recommended to use twisted pair cable with impedance 120 Ohm.

Do not use cables of different types in one and the same measuring line.

If a measuring line cable must be laid in parallel with power cables, use a shielded cable.

Follow recommendations in the table when choosing a particular cable type:

RS485 CAN2.0 Cable cross-section, mm2
Cable length, m Speed, kb/sec Cable length, m Speed, kb/sec
<1200 9,6 <500 100 0,75 – 0,8
<800 19,2 0,6 – 0,75
<400 38,4 <150 300 0,45 – 0,6
<200 57,6 0,32 – 0,45
<100 115,2 <40 1000 0,25 – 0,32

The maximum possible data exchange speed between interface converters and digital modules via RS-485 and CAN 2.0 interface is determined by cable line length, cable resistance per unit length and amount of the digital modules installed.

In order to determine the speed of the connected RS-485 interface device, one can use the program for device detection in the measuring line RS – 485.

The program performs search of the RS-485 devices connected to the measuring line. As an installation wizard one can use any converter of USB-RS 485 type, which is connected to PC and which forms virtual COM-port in it. The program enacts the COM-port specified by the user and creates requests with various exchange speed values, parity bit types and MODBUS-devices addresses. Upon completion of each cycle the program depicts the amount of successful requests, which have been answered by the controlled device.