One of the readers of these pages, Slavo Tomascik, has sent me pictures of a modem card, a true food for my curiosity. It is a beautiful classical piece from the beginning of the "modem boom". All the basic elements of modern modems can be seen, not integrated within a single chip, but almost discretely, so their function can be traced, and the modularity stemming from "layer-ness" of modern communication protocols and methods can be seen. But let's start at the beginning.
This card was manufactured by - once pioneer of modems, later swallowed by one of the "sharks" and nowadays completely forgotten - U.S. Robotics. The card enabled up to 2400 Baud connection based on the V22bis protocol.The first remarkable thing is that the card consists of two "storeys" - the base board, which is the card going to the ISA slot itself; and the modem board atop of the basic board (note also the date - 1991 - and the relatively extensive usage of surface mount devices). This is the first feature of modularity - the base board isolates the modem board from the "harsh" phone line, allowing to adapt the card to different national phone standards in various countries; on the other hand, it provides interface between PC and the modem board. The TL16C550 IC on the base board is a classical UART, so the card appears from the PC side of view as a standard COM port; but it also truly connects to the modem board via serial line, except for absence of RS232 level conversion (which is not needed as the signals don't get outside the PC box). Such a solution enables to reuse the same module in an external modem, which was then an important move towards decreasing develompent and manufacturing costs. Let's now investigate the modem board further:
Except the various "standard" digital ICs of the 74xx type, the board is occupied mainly by a 3-piece chipset by Exar. These are relatively highly integrated circuits - XR2401 (G) is a DSP and XR2402 (F) is its companion analog front-end (AFE, containing DAC/ADC, variable gain amplifiers, clock recovery circuits, etc.) These two circuits together represent the core modulator/demodulator of the V22bis quadrature modulation, i.e. convert the stream of "ones" and "zeroes" into "whistles" passing through the phone line, and vice versaThe third circuit of the chipset - XR2443 (E) - is a 8051 (8052) type microcontroller, with a custom firmware in the internal ROM. This firmware implements protocols one level above V22bis, namely error detection and correction (so called "autoreliable") protocols V42 and MNP2-4, and data compression protocols V42bis and MNP-5. This firmware needs relatively large data fields, stored in an external 32kB SRAM memory (A). Microcontroller (E) is also the linking chain between PC and the modem part - the base board 16C550 UART's signals are connected to this microcontroller. The whole PC-to-modem protocol - AT-commands - is then implemented in the microcontroller, and parameters set via this protocol are stored in an EEPROM memory (D). This part of firmware is not contained in the internal code ROM, but is executed from an external EPROM (B). This is a further step towards modularity: while the firmware implementing standardised and licenced protocols (error correction and compression) is stored in the read protected internal ROM; the never-properly-standardised AT-commands, where vendors like to add various little features, can be easily modified. With advancing integration this type of "modular" modems gradually disappeared, and finally the ultimate flexibility was provided in implementing almost all data processing in PC-software, thanks to the vast computing power of the "main" processor in modern PCs, while the internal modem is nowadays essentially stripped down to the equivalent of the AFE circuit.