Galvanically isolated SpaceWire

The physical signaling layer for SpaceWire is specified in ECSS-E-ST-50-12C as LVDS per ANSI/TIA/EIA-644. LVDS is a current mode, differential interface that offers several benefits over other types of physical interfaces particularly in the areas of noise rejection, electromagnetic noise generation, and power supply decoupling. While LVDS offers a marginal immunity to common mode voltage differences of about 1.2V, systems with physical separation, separate power supplies or a non-common ground reference can exceed this limit. Galvanic isolation, using transformer coupling similar to that implemented by standard Ethernet interfaces, can AC couple the interface and reduce the sensitivity to common mode imparting an increased level of reliability to the overall system. Unfortunately, standard SpaceWire does not readily support such isolation. Ideally, an AC coupled interface has a near 50% bit transition density to keep the reference near the midpoint of the voltage span. The encoding of data and the clock recovery scheme of SpaceWire data on a transformer coupled physical layer presents a reference offset problem since its data is neither randomized nor encoded use a leveling code such as 8B/10B. Worse yet, with SpaceWire's clock recovery encoding, a high transition density on the data line can result in a subsequent low transition density on the strobe line. This paper describes the development and execution of a galvanically isolated SpaceWire network using a transformer isolation approach. The problems encountered, analysis performed, and techniques used to implement a reliable galvanically isolated SpaceWire network are presented and discussed.