Is it necessary to use a crossover cable for connecting the Profinet switch in the industrial PLC domain?

Posted by on

 In the previous article, we mentioned that Profinet has dedicated four-core cables for building networks. However, Profinet cables can be relatively expensive and require specialized connectors. Considering cost factors in some less demanding scenarios, certain manufacturers may opt to use regular Ethernet cables as a substitute for Profinet cables. In such cases, a question arises: should a straight-through cable or a crossover cable be used?

What is a straight-through cable? And what is a crossover cable? To address this question, it's essential to understand the wiring sequence of the cables. We know that the transmission medium for 100BASE-TX in Gigabit Ethernet is twisted pair cables, with each cable comprising four pairs of twisted wires, namely: green pair, blue pair, orange pair, and brown pair. These four pairs of twisted wires can form different wiring sequences. The Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA) has defined four wiring sequences: T568A, T568B, USOC(8), and USOC(6), with T568B and T568A being the most common.

The T568A wiring sequence is: White-Green, Green, White-Orange, Blue, White-Blue, Orange, White-Brown, Brown;

The T568B wiring sequence is: White-Orange, Orange, White-Green, Blue, White-Blue, Green, White-Brown, Brown;

The following diagram provides a visual explanation:












With the concept of wiring sequences in mind, let's discuss straight-through cables and crossover cables. A straight-through cable, also known as a straight cable, is a cable where the wiring sequence at both ends of the cable is the same. For example, both ends are either T568A or T568B. Due to backward compatibility issues, T568B wiring sequence is commonly used.

A crossover cable refers to a cable where one end has a T568A wiring sequence, and the other end has a T568B wiring sequence (different wiring sequences at both ends).

So, when should you use a crossover cable, and when should you use a straight-through cable? Generally, follow this principle: use a crossover cable between similar devices and use a straight-through cable (or straight cable) between different devices. For example, use a crossover cable between two computers or two switches.



Use a straight-through cable to connect the computer to the switch.












Now let's return to the question in the title of this article: Should a crossover cable or a straight-through cable be used to connect Profinet switches? My answer is: both can be used. Why? Because Profinet switches have automatic cable sequence adaptation functionality. They will automatically check the cable sequence and make the corresponding adjustments, so both crossover and straight-through cables can be used for connection. The most commonly used cable in our daily use is the T568B straight-through cable.

After reading this article, do you have a clearer understanding of a concept that might have been a bit unclear before?