Q. Replacing the CMX602A with the CMX602B
A. CMX602A CMX602B Differences.
RXD State in Tone Alert Detect Mode (TAS/CAS detection)
The CMX602A RXD pin is held at Logic 1. In FSK Receive mode, the pin remains at logic 1 when idle or switches between Logic 0 and Logic 1, to reflect the incoming data stream.
The CMX602B RXD pin is held at Logic 0. In FSK Receive mode, the pin switches to logic 1 when the decoder is idle or switches between Logic 0 and Logic 1, to reflect the incoming data stream.
This pin is only used to provide information when FSK Receive mode is enabled and the idle level should not be relied upon in design because noise spikes (that could be interpreted as data) may cause it to transition.
FSK/CAS twist performance
The CMX602B has about 1dB lower twist acceptance to FSK signals and CAS dual tones.
The 1dB difference will not cause a problem in use and is within the requirements of network systems.
Tone Alert (TAS/CAS) and FSK level detector on threshold (sensitivity)
The CMX602B has a slightly reduced sensitivity when used at or around 5V.
Both devices give excellent performance across the range of 2.7 to 5.5 V and the sensitivity is within the requirements of network systems.
Q. I want to develop a product that detects an incoming call during an Internet session. The "far end" modem is automatically silenced by the central office, but the "near end" modem still sends out high level voiceband energy that makes CAS tone detection difficult. Do you have any ideas as to how to overcome this?
A Typically, this type of application is performed in an adjunct box that doesn't have the benefit of a 2-4 wire hybrid to reduce the transmitted "voice" levels. Since the CAS tone is easily overcome by the near end modem signals, an alternate way of detecting the incoming call is to look for the "call waiting beep" (Subscriber Alerting Signal, 440Hz tone). This tone is lower in the voice band and is more easily detected, in the presence of modem signaling, than is the CAS tone.
The FX/MX165C is an ideal part for this function. When operated with a standard "colorburst" (3.58MHz) crystal and programmed to look for a 123Hz tone, the MX165C will actually look for the 440Hz call waiting beep, and can therefore be used to detect incoming calls during Internet sessions.
Please be advised that CML doesn't guarantee the performance of its devices when they are operated outside of their published specifications.