COM/LPT ISA card

This is an another ISA PC card, this time it is a 2xCOM+1xLPT card. It is perhaps the simplest card used in PCs and once it was - together with the videocard - the type of card used the most, each PC contained one. Later, with increasing integration of peripherals onto the motherboard, this type of card become more and more rare. Today, in the mainstream PCs, these ports are considered obsolete and are replaced by USB and similar, faster, but much more complex interfaces. However, they are still used - even in the card form - in industrial PCs, as - mainly the RS232-type COM port - is simple yet very rugged means of communication.

Originally, these cards were simply built up from a UART chip (16C450 and compatible) and a handful of simple 74xxx logic, which performed the address decoding, ISA bus decoupling, LPT-port input and output buffering and the required signalling. Thanks to the high manufacturing volumes, some of these functions have been integrated together and specialised chips have been manufactured - this card is an example of this stage. Later, the complete ISA-interface (replaced then by PCI and its variant LPC), COM and LPT port have been integrated, sometimes together with a floppy-disc controller and other functions, resulting in a chipset or a single chip - so called Super-IO chip, which has then been easily integrated onto the motherboard. Winbond, producer of the main chips on this card, is one of the leading manufacturer of Super-IO chips for the PC industry today.

COM/LPT ISA card

The main part on the card is the W86C451 (G), which integrates a 16C450-compatible UART together with the control buffers of the LPT port, and outputting the 1.8432MHz clock required for the other UART (W86C450, 16C450-compatible (H)) derived from the 14.31818MHz clock present on the ISA bus. The LPT port requires then only an output data latch (B) with terminating RC-network (M, N) to form a complete port on the standard D25 connector (O). The two serial port (L) signals are converted to the 12V levels by 75188 drivers (K) and back by 75189 receivers (J). These chips require to be supplied by 12V, that's why the PC power supply had - and still has - the -12V power rail. There is also a couple of 74LSxx chips, interfacing to the ISA bus.

This is a truly historic card, but well documents the evolution of the PC phenomena.


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