Ok, so not long after I published the article on the hardware teardown of the Seagate Dockstar, I couldn’t help myself so I started working on things to do with this device. I did a lot of research in regards to the capabilities of the Dockstar, including being able to push a customized Linux OS on the device. Once I saw the article at Hackaday that covers exactly how to replace the OS, I knew I had to do it for myself. There are two ways to perform this upgrade however in order to capture syslog output and to be able to get to the bootloader, a serial port is required. Just about all of the sites will describe the pins needed to make the connection, however none of them detail how to do it very clearly and none of them address the issue of aesthetics. Read on for my method of adding a serial port to the Dockstar without affecting the look of the device.
Before We Begin….
The Seagate Dockstar has a serial port available via three of the pins on the header at the front of the PCB. The issue is that they’re not very easy to get to without having to disassemble the device each and every time you need to do a recovery on it. This is hardly an ideal solution, and who knows what I’ll be doing with the device in the future. If I decide to embed the device and something goes wrong, I’ll have to have access to the serial port in order to debug it.
But, simply having access to the serial port is not enough. The Dockstar’s aesthetic elegance is in the fact that it’s so simple. No massive amount of connectors aside from Power and Ethernet, and with little room to begin with, I don’t want to have a cable hanging out of the box just to have access to the serial port. After much deliberation, I decided that a pin-row setup would be ideal instead of some other outward-facing connector. The advantages to a pin-row set up is that there are only as many holes as are needed to establish connection and the connector size is significantly smaller than would be a standard DB-9 connector. An additional advantage to the pin-row setup is that the connection would be temporary and can be easily removed. The resulting connection port would still be cleanly presented and would not stick out like a sore thumb.
In order to pull this off, you will need the following items:
– a CA-42 USB cable. – This is most commonly sold as a Nokia cable through Amazon and can usually be had for a few bucks. This is required as the dockstar’s serial port voltages are at a 3.3V TTL. interfacing it to a standard +12V/-12V serial port will damage your Dockstar. The CA-42 cable has a PL-2303 USB to 3.3v TTL serial adapter in it which provides the required 3.3v TTL and gives an easy to use connector for plugging it into your host PC.
– a 4-pin header with long pins. – The pins have to be long enough that they will go through the Dockstar case and into the matched connector securely.
– a matched connector for the 4 pin header. – This will be mounted inside the Dockstar.
– Heat shrink tubing of various sizes. (Use the images as a guide)
– A couple of spares of the 4 pin header and the connector. (We’ll use one spare for making the holes in the case, but it’s always good to have extras just in case.
– Soldering Iron
– Lighter (for heatshrink)
– small diameter drillbits
– Spudger (or Radio Shack soldering toolkit)
– A Linux machine with an available USB port. Note: It may be possible to use a windows computer for testing however my USB adapter only works in Linux.
Now that you have all the components, it’s important to stop here for a sec and cover the legal mess. It is critically important that you know what you’re doing. You can not blame me or hold this site responsible (or the maintainers of this site) if you do something and blow up your Dockstar. Be careful, do your research, check twice, solder once.
Please note that if you have never worked with shrinkwrap, the important thing is to watch the fire and keep it moving. If you leave the lighter in the same place for too long, the shrinkwrap will stop shrinking and will catch fire. When in doubt, apply the hat quickly and watch the shrinkwrap closely. If it does something wrong, move the lighter away and start blowing on it to cool it down.
Part 1: The Cable
It’s easier to do the modification on the cable first rather than do the Dockstar portion due to the fact that part of performing the Dockstar side of things will require testing to make sure it’s all working properly. So, let’s get started.
This is the cable that we will be hacking together. The pin-row connector shown above is a 4 pin wirewrap terminal and a push-on style PCB mount connector.
In the above photo,you can see the long header pins and the matching connector and how they fit together. Before we get started with modifying the cable, we first need to figure out how it’s wired up. Because there is a very good chance that you have a generic cable, and generic cables are wired differently, we will start off with spudging the USB connector apart.
Follow the plastic seam of the USB connector with the sharp blade of the spudger. Gently work the two halves of the plastic apart until you are able to seperate them. You should see a connector that looks like the one below.
A closer of the PCB will reveal that the wires (white, blue and green) are labeled for our easy hacking convenience . The three wires in my cable are Blue(GND), Green (RxD), and White (TxD). Now that we know which wire does what, reassemble the USB cable as we will not need to do any work on this end of the cable. Starting with the 4 pin connector, pick one of the two internal pins and remove it.
The reason for removing the offset pin is for two reasons. 1) There are only three wires required for connection and 2) The missing pin will allow us to key the connector so that it can’t be reversed. Going back to the USB cable, cut off the fat Nokia phone end and strip the cable back about an inch. To help with soldering, insert the long end of the header pins into a block of breadboard. This will help hold the connection stable while you solder the cable. You will also need to cut the small diameter shrinkwrap in three sizes as shown below.
The reason for the three lengths of heatshrink tubing is that we will build up the edge of the cable to a large size so that we can use the larger heatshrink tube (in the back of the picture) to bind the header pins into the wrap and the wrap to the end of the cable to strengthen the cable. If you have never worked with heatshrink tubing, it’s very easy to work with. Start with the longest piece of tubing, and slide it over the cable. Make sure that the cut end matches the end of the insulation and heat with the lighter. KEEP THE FLAME MOVING ACROSS THE HEATSHRINK!! Once the heatshrink has stopped shrinking, allow it to cool and repeat for each piece.
This is the end result of the shrinkwrapping. Now that the end is built up, slide a piece of the large shrink over the end of the cable but do not apply heat just yet. Strip back the individual wires so that you can attach them to the header pins.
Now that we’re ready to solder the connector, it’s important to decide how to create the pinout. In my setup, I elected to have the GND connection by itself, then the TX and RX pins. For each wire, wrap the wire around the soldering post on the header pins and solder. Be sure to use only enough solder as is required for the connection and do not bridge the pins.
Now with the cable soldered and the connections solid, it’s time to apply the heat to the large heatshrink. Very carefully pull the end of the heatshrink over the black plastic header and gently apply heat. Adjust if needed and let the tubing shrink without it pulling itself off of the header plastic.
When you have it this far, go ahead and apply heat to the rest of the heatshrink tubing, making sure not to singe it. When you are done, you should have a cable looking like the one below.
This was the easy part, Now it’s time for the dockstar.
Part 2: The Dockstar
We’ve got the cable, but without something to connect it to, it’s pretty useless (unless you want to use it on a breadboard). Let’s take a look at what’s going on.
Since I’ve already determined that I want the connector for the serial to come out of the back of the device and I’ve already found a suitably small connector, it’s time to find a location where I can attach it without getting too involved or without interfering with the existing ports on the back of the dockstar. I’ve elected to put the serial port just above the center USB connector. In the photo above, you can see how much space we’re dealing with in comparison with the USB ports and the header socket.
Some of the RF shielding will need to be removed, but thankfully the metal is pretty flimsy and easily cut. Be sure that when you remove the little fins pictured that you do not distort the top of the metal shielding. We need a surface as smooth as possible for the superglue to properly bind with the connector for the serial port.
Here is the picture of the unneeded shielding removed. Please only remove the shielding that you need. Next, you will need to prep another three pin header just like you did for the cable. Rather than trusting faulty measurements and guessing, we’re going to use the header’s pins themselves to point out where we need to place our holes.
Using a pair of needlenose pliers and your soldering iron, heat the pins evenly and apply moderate pressure. The pins may wiggle slightly but don’t let them move too far off otherwise your holes won’t be lined up. When all three pins have punched through, remove the pins with the needlenose pliers. Use the small diameter drill bit to widen the holes and to furr out any residual plastic scraps. Your finished holes should look something like below.
Just to make absolutely sure, go ahead and test with the connector and the cable to make sure everything fits properly.
If you’ve made it this far, you’re doing good. Now it’s time to prep the connector for supergluing into the Dockstar’s case. Since it’s a four pin connector and we’re only using three pins, make sure that the connector is properly oriented so that it fits properly and so that the serial cable can move freely in and out of the connector. Back the cable off a bit so you can see which pin is missing and cut off the connector’s matching pin to eliminate a possible mis-wiring later on. Go ahead and attach and solder a wire to each of the three remaining pins.
In order to glue the connector socket in, take the excess wire and coil it up for now. Apply a thin coat of superglue inside the Dockstar and when aligned, push the three pin header you used for burning the connector in through your drilled holes and into the connector. This will hold the socket steady while the superglue cures. Give it about 15 minutes to cure properly, then gently remove the pins from the socket. At this point, you should have a fully mounted socket like in the picture below.
Now that the socket is taken care of, we need to attach it to the serial port on the Dockstar’s board. Before we do anything permanent, we will test the serial port and then once we are satisfied that it’s all working, we’ll solder them in and close it up. Start off with wrapping the ground wire to the lower right hand pin on the connection block. This is the common GND connection and must be established first. If you are using my wiring plan, the GND wire is the single pin by itself on the three pin header we made earlier.
The two pins to the left of the GND pin are the RX and TX pins respectively. Attach the centermost pin to RX and the remaining pin to TX on the superglued connector.
Remember, we have NOT soldered the three wires yet. Also, make sure that none of the wires are touching prior to connection. In order to test the serial port and make sure we have it hooked up right, connect the three pin header on the serial cable to your socket on the back of the Dockstar and connect the USB connector to your computer. Do not apply power to the Dockstar yet. Open up minicom and set the serial device to /dev/ttyUSB0, 115200, no parity, 8 bits, 1 stop bit. When properly configured, apply power to the Dockstar and watch your console window. If you got it right, you should get similar output like below. If not, pull the power plug on the Dockstar and disconenct the serial cable out of the back of the dockstar. Swap the left two pins (RX and TX) and try again. You should get output like below.
Now that you’ve tested the wiring, disconnect the power and the serial cable from the Dockstar. Solder the wires in place and reassemble the Dockstar. Be careful closing the cover as you want to make sure that the wires coming off the superglued socket do not touch the metal shielding. (Editor’s Note: I really need a hot glue gun)
Close it all up and test it one more time. If everything works as should, you’re good to go. Now you can access the serial port without having to take your Dockstar apart over and over again.
Here is a picture of the completed serial cable mod. The serial cable is plugged in right above the keydrive in this photo.
Same view as above, but with the serial cable removed. Nothing but three holes.