This guide will demonstrate how to connect to the internal serial UART (Universal Asynchronous Receiver/Transmitter) port to gain console access for diagnosis and testing purposes.
WARNING: THE FOLLOWING PROCEDURES MAY VOID YOUR WARRANTY OR BRICK YOUR NAS - USE EXTREME CAUTION AT ALL TIMES
BASIC COMPUTER SKILLS AND SOME FAMILIARITY WITH ELECTRONICS ARE REQUIRED
Never try to interface with hardware unless you have the proper experience and tools for the job. One wrong move and the hardware can be permanently damaged.
The following instructions are based on the WD My Cloud PR4100 NAS, but may be applicable to other WD My Cloud models, possibly with minor variations applied as needed.
PREREQUISITES: A 3.3 Volt USB to Serial Breakout Board and appropriate connectors are required. A good multimeter is required, but an old analog multimeter will often work just fine. A decent terminal emulator program is also a must, with PuTTY being one of many good programs to use.
Shut down the NAS and remove the power cord, before removing the outer case, which is held on by three phillips head screws located on the back panel. The outer case slides back, then up. Be very careful not to hit any of the internal components.
The internal UART port is fitted with a standard JST 4 pin connector. These connectors were commonly used on the audio interface of older CD ROM drives. With the front of the NAS oriented to the right, the pinout of the internal UART port is (GND, TX, RX, VCC). This is may change, so one should ALWAYS VERIFY that the pinout is correct before trying to make a connection. Only the GND, TX and RX pins are required. Leave the VCC pin disconnected or damage to the NAS motherboard may occur.
To verify the pinout of the UART port, first identify the GND pin by measuring the resistance between each pin and a known grounding point such as the metal frame or a heat shield. The power plug of the NAS must be removed to measure resistance. The TX pin can be identified using an analog voltmeter to measure the voltage of each pin as the NAS is booting up. The voltage should vary slightly, before finally settling at around 3.3 volts. The RX and VCC pins can be a little more difficult to identify without actually testing for a connection. The problem with the RX pin is that its voltage can vary from device to device. Sometimes the RX pin is pulled high (3.3 volts), sometimes it’s pulled low (around 2 volts), and sometimes it’s left to drift until data is sent… making the voltage vary wildly. The VCC pin will tend to have a rock solid 3.3 volt reading, and it often has slightly less resistance than the TX and RX pins when measured against ground.
Again, the power cord must be removed when measuring resistance and one should be very careful to avoid shorting any pins while measuring voltage with the NAS powered on. A USB Logic Analyzer is another way to identify the TX, RX, and VCC pins. However, one has to know how to interpret the readings it provides, which is beyond the scope of this guide.
To connect the 3.3 Volt USB to Serial Breakout Board to the internal NAS UART port, use the following as a guide for making the connection. Note that the TX and RX pins must be crossed for communication to take place. NEVER connect the VCC pin or damage to the NAS motherboard may occur.
GND <--> GND RX <--> TX TX <--> RX VCC <--> NO CONNECTION
After the pinout has been verified and a physical connection has been made, the terminal emulator program must be configured to establish a serial connection using the following parameters. The COM port is determined by device drivers, which can be viewed using the OS Device Manager.
Bits Per Second: 115200 Data Bits: 8 Parity: None Stop Bits: 1 COM Port: COM5 (Determined by OS device drivers)
After all connections and settings have been verified, open the terminal emulator program and power on the NAS. If a successful connection has been made, a series of boot messages should start appearing in the terminal emulator program window. Eventually the opportunity to enter a Linux command prompt will appear.
When finished, shut down the NAS and remove the power cord BEFORE removing the physical USB to UART connection and/or replacing the outer cover.
Lastly, this procedure should NEVER be attempted without the proper skills and a good reason for doing so. While it can be used as a means of unbricking the NAS or for testing purposes, it should not be taken lightly. The risk of permanent damage to the NAS is VERY REAL, so one should always use EXTREME CAUTION.