Why does the external loopback on the DS26401/DS2155 not work properly, though the remote and payload loopbacks work fine?
A8. The cause for this error could be that you are not using the correct source for the transmit clock (TCLK) in the DS26401. The DS26401DK was designed with three major components: a LIU, a framer, and a FPGA for specific signal multiplexing. Unfortunately, the receive clock from the LIU goes to the DS26401 and a test-point header, but not to the FPGA. Therefore, to perform an external loopback, you need a way to connect RCLK to TCLK. Solution One Manually jumper RCLK and TCLK on the DS26401DK with the test-point headers. Make sure the TCLK register in the FPGA is three-stated. This is the default value. Solution Two You can control the source on the transmit clock with TCR3 register (TCR) using Bit 4 (TCSS0) and Bit 5 (TCSS1). Set TCSS0 and TSSC1 to a logic “1”. This will cause the transmit clock to be the same as RCLK from the LIU.
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