18 Nov 2013: First revision PCB layout progress
Although it is not yet complete this is an almost beautiful design, with a few interestingly-scary areas around the Ethernet connectivity to the 4-port Gigabit RJ45. In the end individual transformers were chosen with centre taps, as the layout could be varied. As there are 5 ethernet ports (one for EOMA68) and there are 4 sets of wires for Gigabit that's 20 transformers! It made for some interesting layout issues given that this is a 4-layer PCB.
Also after talking with people on the list it was decided to add support for POE. The Ag5300 was chosen because it is capable of easily supplying 15W if used in 802.11af mode. Going up to 802.11at (30W) was considered not to be worth sacrificing the pins: there are only 4 GPIO pins left.
Size-wise, the addition of the POE module (60mm long and 18mm wide!) gave some space issues which required a small connector shuffle. USBx2 has moved into the top left corner, along with the USB Power IC and the USB Hub. RS232 moves onto the top edge (right). POE module is just below that, taking up an enormous amount of space due to the size of the heat spreader areas as well as two Bridge Rectifiers. Also added were four mounting holes. USB, 4xGbE and RS232 were all made to "overhang", bringing the unit size to around 11.5cm x 10.3cm with the PCB being slightly smaller. If the overhang of the Power and VGA ports are taken into consideration the unit size is 12cm x 10.3cm.
This PCB layout is, surprisingly, almost ready to go. The placement and layout of the MiniPCIe needs to be double-checked; an alternative SIM slot needs to be chosen; the power plane needs to be done, and that's about it, amazingly.
In essence, this router I/O board covers the options that the Flying Squirrel did not. It can be cut-down (removal of the 4-port GbE) and audio put in its place, and that would make a very good Desktop PC. Perhaps with the addition of an extra 3 or 4 USB ports. Still up for consideration is whether to include an ultra-low-cost PIC or Embedded Controller of some kind. Photo below can be viewed stand-alone (View Image in browser) - it is bigger than shown below, so some of the detail of the connections can be seen clearly.