Using The "Snubber"
U.S. transformer-based DAAs can take advantage of the RC network or "snubber" normally found across the hook switch to couple the Caller ID message to the transformer. Using a 2.2-kΩ resistor and a 0.15-µF capacitor, the snubber also supplies a path for Caller ID (Fig. 6). With this approach, no additional parts are needed to switch in a signal path. Adding a switch and a capacitive path to get the caller ID message across the hook switch may be necessary for transformer-based DAAs in other countries.
Line-in-use and parallel pickup are features usually required in applications that will have a modem making unattended connections on a line shared with a telephone. The telephone should always have the highest priority on the network, so the modem shouldn't try to make a call if the line is in use, and it should relinquish the line if another device picks up the same line. Not all of the listed isolation methods normally support these features.
If this is a requirement for an application, selecting the isolation method should take that into consideration. Also, in the case of the Infineon DAA, the output of a voltage-to-frequency converter represents the line voltage. To determine the line status, the modem or host must be able to use this signal as an indication of the line voltage. This requires a counter function capable of monitoring the line status during the on-hook period.
When deciding on the modem design, it's important to look at all of the variables before deciding on what approach to adopt. Make sure that you understand how all of the features you require are supported and where you need to concentrate your engineering. Make certain that all circuitry necessary to support the required features are accounted for when comparing technologies. Don't be enamored by "new" techniques when "old" technology, like the tried and true transformer, may be better suited for your application. See if the level of support from your modem and DAA suppliers is compatible with your requirements. Don't hesitate to ask DETAILED questions while exploring the proper way to use a vendor's products, especially in the area of the DAA because that's where the certification testing will make or break the modem design. If you can't get the level of support that you feel is needed, perhaps you chose the wrong vendor. Including a modem in your design shouldn't be a painful process.
Using The "Snubber"
U.S. transformer-based DAAs can take advantage of the RC network or "snubber" normally found across the hook switch to couple the Caller ID message to the transformer. Using a 2.2-kΩ resistor and a 0.15-µF capacitor, the snubber also supplies a path for Caller ID (Fig. 6). With this approach, no additional parts are needed to switch in a signal path. Adding a switch and a capacitive path to get the caller ID message across the hook switch may be necessary for transformer-based DAAs in other countries.
Line-in-use and parallel pickup are features usually required in applications that will have a modem making unattended connections on a line shared with a telephone. The telephone should always have the highest priority on the network, so the modem shouldn't try to make a call if the line is in use, and it should relinquish the line if another device picks up the same line. Not all of the listed isolation methods normally support these features.
If this is a requirement for an application, selecting the isolation method should take that into consideration. Also, in the case of the Infineon DAA, the output of a voltage-to-frequency converter represents the line voltage. To determine the line status, the modem or host must be able to use this signal as an indication of the line voltage. This requires a counter function capable of monitoring the line status during the on-hook period.
When deciding on the modem design, it's important to look at all of the variables before deciding on what approach to adopt. Make sure that you understand how all of the features you require are supported and where you need to concentrate your engineering. Make certain that all circuitry necessary to support the required features are accounted for when comparing technologies. Don't be enamored by "new" techniques when "old" technology, like the tried and true transformer, may be better suited for your application. See if the level of support from your modem and DAA suppliers is compatible with your requirements. Don't hesitate to ask DETAILED questions while exploring the proper way to use a vendor's products, especially in the area of the DAA because that's where the certification testing will make or break the modem design. If you can't get the level of support that you feel is needed, perhaps you chose the wrong vendor. Including a modem in your design shouldn't be a painful process.