Internal faxmodem card

With the spreading of broadband internet and mobile communications, the classical phone line modems seem to extinct; but they still represent a viable and low-cost connectivity for a variety of applications, including but not limited to data collection, remote upgrading and control and various industrial applications.

This is an internal ISA-bus faxmodem card of the middle-90ties, when the advance of technology allowed to integrate a 14.4 kbit/s (V.32bis) transfer rate modem into two or three custom chips. This card is based on a Cirrus Logic chipset, which fulfills the functions needed for the modem operation: first, there is a controller, which interfaces to the ISA bus via which it simulates function of a standard UART (this is for compatibility with external modems which connect to PC via true serial ports). The controller answers to the host (PC) issuing the AT-commands and acts upon them. When the host issues the dial command (ATD), the controller uses the analogue circuitry which interfaces the phone line to simulate headset pickup and to dial the required number. When the called party picks up, the controller commends the attached DSP to create a series of frequencies, using which it negotiates the communication protocols to be used (this is the well known "screaming" while the modems are connecting). The DSP calculates the necessary voltage levels and uses an analog front-end (combination of DA and AD converters and the phone line interfacing circuitry) to send and receive the signal with varying frequency and phase, carrying the transmitted data. It then communicates the data via the controller to the host in both directions.

internal faxmodem Cirrus 14k chipset

There is nothing to disassemble on an internal PC card, so we can see the abovementioned function blocks right away: the chipsed containing the controller and the DSP (A and C), a SRAM as a working memory (B), a serial EEPROM (G) into which the controller stores the modem settings, the analog frontend (SAFE chip) (D), and the analog circuitry (E) which interfaces the phone line (F).

This modem card may be obsolete, but is still in good working condition and is usable for example in conjunction with an older PC as a standalone data collection and forwarding system.


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