Keep that front panel shut!

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The Pundit-R from Asus is a great small form factor computer for use with MythTV. One of the biggest downsides is that the SPDIF output is on the front panel, behind the door. If you want to use the SPDIF, you have to keep the front panel open. Here is a quick step by step on how to add a SPDIF output on the back panel so you can keep the front panel shut when not in use.

The standard disclaimer applies here: I'm not responsible for any damage you cause to your computer, stereo, or yourself. Surely this voids your warranty, make sure your computer is functional before you muck around with the motherboard. There might exist some high voltages inside your computer, unplug before working. Make sure you have a static free work environment. You will need to solder, so if you don't know how to, you may want to get someone who can or just deal with the front panel being open... YMMV statement: I've verified that this works with my receiver (Sony 9000ES) playing MP3s at 48kHz. The transmitter is rated for 25Mb/s and should handle anything thrown at it.

I decided to use a separate regulated supply for the new SPDIF transmitter because the one on the motherboard had only 2.1v going to it's supply pin. That is too low for the Toshiba transmitter. I stole power from the firewire connector, which is more convienient than taking power from a drive cable. The transmitter uses roughly 15-30mA, so there's still plenty of power to spare. Of course, if you want to fish around the board and find 3.3v, you can omit the regulator.

If you're really budget conscious, you can try the TOTX147L, it's only $1.50 but has a maximum bit rate of 15Mb/s which should be enough as well. I went with the higher speed one since it wasn't that much more and it saved me from having to try and figure out what bandwith was actually required. (I suspect 15Mb/s is more than enough for even surround sound).

Items you'll need:

• Low wattage soldering iron (15W or so), solder, rework wire.
• CD-ROM audio cable or similar jumper cable
• two 3-pin 0.100" headers
• TOSLink Transmitter, TOTX142L (available from Digi-Key You may also be able to use the TOTX147L, it's only $1.50 vs $5.00 for the TOTX142L.
• 3.3v regulator, I used a UA78M33CKTPR, also available from Digi-Key
• 0.1uF capacitor, any kind, I used a 0805 which fit perfectly between the output and ground pins on the regulator.
• some sort of board to wire your circuit on
• miscellaneous hardware (screws, plates, etc)

Here's a list of the Digi-Key part numbers and prices.

qty	part number	description	unit price	amount
1*	296-13425-1-ND	UA78M33CKTPR 3.3v Regulator	0.56	0.56
1*	TOTX142L-ND	TOSLink Fiber Optic Transmitter	5.00	5.00
1*	490-1725-1-ND	0.1uF 0806 Ceramic Capacitor	0.57	0.57
1	929834-03-06-ND	6-pin 0.100" single row header	1.04	1.04
* items I actually ordered from Digi-Key, others are for reference, since I already had some on hand. Please double check.

Here we go (click on the pictures for larger versions):

1) Disassemble your Pundit-R. I assume if you are technical enough to perform this operation, you will be smart enough to figure out how to get the motherboard out of the case. Here are some tips in case you get stuck: tip: remove the power supply first, it'll give you that extra space to slide the motherboard out. tip: remove the rear panel that holds the serial port and tv-out connectors. There are 3 screws that hold it in place. If you don't remove this panel, you'll never get the motherboard out. tip: You don't need to remove any of the nuts which are attached to the ports.
2) Flip over the motherboard and locate the SPDIF connector: (it's between the two firewire ports)
3) You'll want to take your 3 pin header (if you got a 6 pin one, then break/cut off 3 pins and save the rest for later) and trim the middle pin so that it doesn't touch the middle pin of the existing SPDIF connector.
4) Solder a little piece of rework wire to the middle pin. (enough to get to pin 1 of the firewire connector to the left (the square pad)).
5) Lay the header down as shown in the picture and solder the two outside pins to the pins of the existing SPDIF transmitter. Also, solder the rework wire to pin 1 of the firewire connector on your left (the square pad). I put a small dab of hot glue between pin 2 of the header and pin 2 of the transmitter so that there wouldn't be a chance of accidental shorting.
6) If you use a CD-ROM audio cable, you'll have to move some conductors around, since most CD-ROM audio cables have grounds in the middle and usually they are shorted together. My cable has the configuration (black), black, red, white. Use a small pin or paper-clip to release the pin. (push the pin gently through the squares to release the connectors. On the cable that I used, the squares are in the middle of the body, as shown in the picture)
7) Wire up the schematic below using your favorite method. tip: you can easily fit the 0.1uF cap between the ground pin and the output pin. tip: the TOTX142L datasheet says to keep the cap within 7mm of the device. refer to the datasheets, available at Digi-Key, for correct pinouts. datasheet:TOTX142L datasheet:UA78M33CKTPR
8) If you have space on your PCI backplane, then you can attach your SPDIF output to a blank filler plate. tip: I put mine towards the middle, but you should put it towards the top of the filler plate for more stability.
9) Plug in the cable to connect your new SPDIF transmitter. double check your wiring. you should get around 12v at the input of the regulator. you should get 3.3v at the output of the regulator.
10) Double check your wiring...enjoy...donate ;)

(c) 2005 Pat Tjin. No duplication without consent.