Serial and JTAG port LS-XHL
From NAS-Central Buffalo - The Linkstation Wiki
Adding a Serial and JTAG port to the LS-XHL
You have to remove R253 and R254 (see pictures below). These resistors are solely for obfuscation. R253 hides the RXD and TXD vias before AC coupling. R254 hides the pads for 0R resistors for JTAG TMS and TDO and make it look like the traces weren't interrupted.
For a (non AC coupled) UART port you have to short the vias underneath R253 with a 0R resistor, shown in the following image:
For the JTAG to be functional, you have to remove the solder resist from the pads underneath R254 and stuff 0R resistors (red rectangle).
Note: Don't stuff the "SoC BSCAN TMS" resistor (green rectangle), the image is wrong here! This will enable the SoC boundary scan chain, but you have to remove the other resistor which connects the JTAG TMS pin to the CPU TMS pin. These are mutually exclusive.
The LS-XHL also exposes the SPI signals (all but one..). SPI_DOUT is only available if you stuff a DNP resistor under the SATA connector.