Internal high-speed faxmodem card

This is an another internal ISA-bus faxmodem card, based again on a Cirrus Logic chipset, but this time a newer model, capable of 56 kbit/s operation under the V.90 standard.

internal faxmodem Cirrus V.90 chipset

Although some 5 years younger than the V.32 model, the setup is very similar - there are 2 main chips, shared memory, and the same SAFE (analog front-end, DAC/ADC combo) chip (CL-MD1724). However, the V.90 technology is much more computing intensive and also demands more hardware - besides a fast (20ns) SRAM (C) shared between the controller (A) - which is ARM based, as the print on it proudly announces - and the DSP (F), but the latter has attached two extra SRAMs (G) necessary for the V.90 mode. The firmware is stored in FLASH (E) (rather than maskROM), hence can be updated ("reflashed") if necessary. One thing which is common between both modems is the serial EEPROM (B), used to store the configuration parameters.

The usage of two SAFE chips (H) indicates (according to the CL-MD56xx chipset datasheet), that this card includes a full-duplex speakerphone feature, to be connected to jacks (L); so utilizing the voice functions of the modem a complete phone/answer machine/fax/modem combo can be built, if the appropriate software is available. The phone line connector (and the connector for the parallel phone)(M) is interfaced via transformer (I), relay (J) and appropriate conditoning and protection circuitry. Of course, the "screamer" is not missing here, too (K).

schematics of faxmodem Cirrus V.90 chipset

Modem cards look simple, but they involve quite complex circuitry both analogue and digital and a relatively massive computing power. These ISA cards were equipped by controllers (here, a 32-bit RISC - ARM) and DSPs. Later, the increased computing power of main x86 processor and the increased throughput of the PCI bus together with the quest for the lowest cost lead to a different solution, where the PCI modem cards are equipped only by the DSP (or even less, only by the analog frontend), and the appropriate computing is provided by the x86 processor.


This article was originally written in 2006, when DSL just started to be introduced as a means for connecting to Internet in this country. In those days, this kind of modems was the norm for connecting to internet from a home; companies often used ISDN connection. In the few years since then we've seen such a rapid revolution in this field, that five years later, in 2011, landline modems are an obsolete and almost forgotten piece of equipment.