Building an Embedded Asterisk PBX Part 2

Introduction

In Part 1 we described the architecture of an IP-PBX and the pros and cons of x86 PC based systems versus embedded systems. In Part 2 we will examine the hardware design at schematic and PCB level, then take a look at the device drivers needed to connect the hardware to Asterisk running on the Analog Devices Blackfin.



Figure 1 shows the completed IP-PBX hardware. The PBX is built around a Blackfin STAMP development card, available off the shelf from Digikey for around $170. Sitting on top of that is a daughter board which contains interface hardware and a SD card socket. Into the daughter board plugs FXO or FXS modules, one for each port. In this example there are two FXS modules on the left, and two FXO modules on the right. The color of each LED indicates the type of module inserted.

This modular design was chosen for flexibility however other configurations are also possible. For minimum cost the entire IP-PBX could be integrated onto one PCB, for example a 4-FXO port design could serve the typical small office using 8-12 IP-phones as extensions.

In the next section, we will take an in-depth look at the design of the modules themselves. The hardware designs (including schematics and PCBs) are open, and can be downloaded from http://www.rowetel.com/ucasterisk

Line Interface Hardware Design

Here are the schematics for:

+ FXS module: http://www.rowetel.com/ucasterisk/downloads/fxsmod-0.32.pdf
+ FXO module: http://www.rowetel.com/ucasterisk/downloads/fxomod-0.32.pdf
+ Daughter board: http://www.rowetel.com/ucasterisk/downloads/4fx-0.32.pdf

Just a reminder on the telephony jargon:

+ FXO ports connect to telephone lines and the exchange.
+ FXS ports connect to analog telephone handsets.