The question of how to safely connect to the telephone line comes up often on this forum and so I want to provide some information on the subject. The circuit configuration that is required to do this function is called a DAA (Data Access Arrangement).

If a circuit like this is designed into a product and the product is offered for sale then the unit must carry agency type approvals to ensure that is has been designed and produced in a manner that meets standards of safety to the users and protection of the the public telephone network. Historically it has generally been required for a product be type approved in each separate country within which the product has been offered for sale. As you can imagine this has been an expensive proposition for many companies. In more recent times there has been a move toward more uniform specifications and today it is generally possible to have a product certified by testing at a single laboratory and then simply make application for approval to each country by submitting the proper application forms, fees and attaching copies of the test reports

There are many ways to devise a DAA circuit but the general considerations are:<dd>

Circuit isolation between the telephone network and the local equipment.

High voltage suppression to protect the local equipment due to surges on the local network line.

Capability to detect ringing signal from telephone line

Ability to switch the telephone line on-hook and off-hook. (i.e. loop current control)

Couple to the line to access the low level audio signal on the line (i.e. listen)

Couple to the line to permit an audio signal to be placed on the line (i.e. talk)

</dd>

I won't show how to design a DAA here, but will instead provide links to web located reference designs that include schematics. When you look at these reference designs it will become apparent to you that it is generally considered possible to design a DAA circuit that is suitable for all European applications, and another configuration that will meet North American requirements. Circuits tested to either one or the other of these two broad categories can generally be registered in most countries.

This first link is to a Philips Application Note AN812 that shows circuit configurations for a DAA and is a great source of information on DAA's in general.

The next link shows a Technical Note #88 from Midcom. Midcom is a transformer manufacturer and is trying to promote a small sized high performance coupling transformaer on this document. I included it here because this includes a schematic for the 'telephone hybrid' function. The hybrid function is also known as a 2 wire to 4 wire converter. Its job is to convert from the full duplex (2 wire) audio path of the phone line to separated transmit and receive audio paths. Note that if you look at a modem design this circuit is most likely not built discretely but instead inside the modem chipset, but here you can see the circuit and how they are built.

This third link is to a schematic for an Opto Coupled DAA from Clare Inc. Clare has produced a special chip that incorporates much of the DAA functionality into a single package. They use optical coupling techniques to eliminate the use of a tranformer. It is a nice solution if small size is important. You can get more informatin on this technology at their web site www.clare.com and search for data on their part number CPC5610. Do note that the Clare part is not a complete one chip fits all solution. You will note in the schematic that many additional discrete components are required in addition to the chip itself.

Lastly I give a link to the CML Reference Design for V.22 Modem. This document will repeat much of the informtion that has already been referenced but it does include two additional items of interest. One is a discussion of DAA Layout Guidelines as pertain to the PC board layout in particular noting the 2.5 mm isolation barrier needed for spacing between telephone network circuitry and local equipment circuits. This document includes a discussion of Caller ID which is unique in that the DAA must provide for an audio path to pick off the FSK CID signals while the DAA curcuit is on-hook.

I hope this information is a useful guide in getting your 8051/8052 project connected.

Michael Karas