除 PMA 上可用的局部环回外，40G/50G PCS 还提供测试模式和远程环回功能，如下图所示。 图 1. PCS 环回 Page-1 Sheet.1 Page-1 Sheet.3 lane distribution lane distribution Sheet.4 Sheet.5 lane distribution lane distribution Sheet.6 Sheet.7 descrambler descrambler Sheet.8 scrambler scrambler Sheet.9 Sheet.10 Sheet.11 decoder decoder Sheet.12 encoder encoder Sheet.13 Sheet.14 Sheet.15 pattern generator pattern generator Sheet.16 pattern checker pattern checker Sheet.17 RX RX Sheet.18 Sheet.19 test pattern test pattern Sheet.20 TX TX Sheet.21 RXD RXD Sheet.22 TXD TXD Sheet.23 remote loopback remote loopback Sheet.24 curve.28 Left Brace Right Brace Sheet.28 XLGMII/50GMII XLGMII/50GMII Sheet.29 Sheet.30 Sheet.31 X16604-030817 Sheet.32 Sheet.33 Sheet.34 X16604-030817 启用之后，发射的测试模式为连续空闲字符，然后对这些字符进行扰码处理。