Automotive Ethernet Compliance: Test Setup Overview

Figure 1: Typical test setup for Automotive Ethernet
PMA compliance test

Our last post, an overview of the five test modes for Automotive Ethernet electrical compliance testing, prepared us for a deeper look at the compliance tests themselves. But before diving into details on the differential electrical compliance tests for Automotive Ethernet, be it BroadR-Reach or 100Base-T1, it might be helpful to take a look at the setup for this endeavor.

Figure 1 provides a overview of a typical test setup for PMA compliance testing. The test fixture in the photo (Teledyne LeCroy's TF-ENET-B) provides an RJ45 breakout section that makes the various signals from the DUT more accessible. These signals come to the fixture from the DUT over a short run of twisted pair. This short cable can be thought of as the interface between whatever output connector the DUT provides (the MDI, or Medium Dependent Interface) and the RJ45 input on the Ethernet test fixture. The test document refers to this interface as a "short" cable but does not specify a length. We'd recommend keeping it as short as possible.

Figure 2: A closer look at the TF-ENET-B
Ethernet test fixture and its RJ45 breakout section

Figure 2 gives us a closer look at the Ethernet test fixture and the RJ45 breakout section that provides direct access to the signal lines coming in from the DUT. While an RJ45 connector has eight signal lines in four pairs, Automotive Ethernet uses only a single twisted pair for data transmission (it's worth noting that the "-T1" in 100Base-T1 means a single twisted pair). When you map those two signals to the RJ45 connector, they must correspond to a signal pair in the connector. It doesn't matter which pair you use, but you cannot use one line from one pair and one from another.

In the case of the test setup depicted here, it's pair 1 that's being used in the test fixture's RJ45 breakout section. Figure 2 shows two blue SMA cables attached to this pair of ports. These SMA cables will bring the differential signal to the oscilloscope inputs.

So that's a short overview of the test setup. Next time, we'll begin looking in more detail at the PMA compliance tests themselves.