Cable Line Length

What is the project cable length taking into consideration the distance to the interface converters location?

The maximum possible length for RS-485 and CAN2.0 networks is 1.2 and 0.3 km respectively. However, one should take into consideration that in this case the data shall be transmitted at low speed. Another factor is the overall impedance of the line – it should not exceed 100 Ohm, while the power supply network impedance level should provide sufficient voltage (over 9 V) for the module, which is located at the far end (from the interface converter) of the measuring line.

It is strictly forbidden to use cables of several types within a single measuring network.

In the case if it is necessary to place measuring line cables along with power supply cables, it is necessary to use shielded cables.

Dependence of data transfer speed on cable length is shown in the table below:

RS485		CAN2.0		Cable cross-section, mm2
Cable length, m	Speed, kbps	Cable length, m	Speed, kbps
<1200	19,2	<300	100	0,6 – 0,75
<400	57,6	<150	300	0,45 – 0,6
<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.