1 Introduction

IEEE Std 802.3cg™-2019¹ defines a 10 Mbit/sec Ethernet PHY that can support up to at least 8 nodes at a total bus length of up to at least 25 meters. The PHY in the Microchip LAN8670/1/2² and LAN8650/1³ are designed to the IEEE standard and can certainly support 8 nodes at 25 meters. When used with an appropriate system of cable and connectors, the products can support many more nodes and much longer reach.

The standard does not specify the exact cable, although it does specify cable characteristics for a specific cable length. It also does not specify connectors, but these, as well as the network topology can determine whether the system can work reliably. This application note will show how the cable, total length, and bus topology determine the inherent noise of the mixing segment. Inherent noise reduces the noise margin of the system and determines whether a system can operate reliably in the appropriate noise environment: inherent noise determines how many nodes can be supported and how long the mixing segment can reach. This application note will show that with good cable, it is possible to design mixing segments to support well beyond 8 nodes and 25 meters.