The internal I/O ports
As mentioned, the USB is going to become the main bus for low-speed devices. But so far we still use the
internal "face" of the ISA bus for a range of purposes. At any PC motherboard you find these:
l The floppy controller
l The serial portsl The parallel port(s)
l The keyboard controller
They all occupy IRQ's which is a central part of ISA architecture and a pain in the a... Let us take a moment
to look at these ports and controllers.
The serial ports
Serial transmission means to send data from one unit to another one bit at the time. The PC architecture
traditionally holds to RS232 serial ports. The RS-232 standard describes an asynchronous interface. This
means that data only are transmitted when the receiving unit is ready to receive them:
The serial ports are controlled by an UART chip (Universal Asynchronous Receiver Transmitter) like 16550
AFN. This chip receives bytes from the system bus and chops them up into bits. The most common
package is called 8/N/1 meaning that we send 8 bits, no parity bit and finally one stop bit. This way one byte
occupies 9 bits:
The serial ports are controlled by an UART chip (Universal Asynchronous Receiver Transmitter) like 16550
AFN. This chip receives bytes from the system bus and chops them up into bits. The most common
package is called 8/N/1 meaning that we send 8 bits, no parity bit and finally one stop bit. This way one byte
occupies 9 bits:
A Guide to Adapters and I/O units.
The serial transfer is limited to a speed of 115,200 bits per second. The cable can be up to 200 meter long.
The serial ports can be used to connect:
l The mouse
l Modems
l ISDN adapters
l Printers with serial interface
l Digital cameras
l ....
These units are connected to the serial ports using either DB9 or DB15 plugs.
In a few years time all these devices will connect to the USB bus instead.
The parallel port
Parallel transmission means that data are conducted through 8 separate wires - transmitting a full byte in
one operation. This way the parallel transmission is speedier than the serial, but the cabling is limited to
5-10 meters. The cable is fat and unhandy, holding up to 25 wires and the transmission is controlled
according to the Centronics standard.
Most printer manufactories use a 36-pins Amphenol plug, where the PC's parallel port holds a 25-pinned
connector. Hence the special printer cable. To the left you se the 25 pin connector, to the right the 36-pin:
A Guide to Adapters and I/O units.
As mentioned, the USB is going to become the main bus for low-speed devices. But so far we still use the
internal "face" of the ISA bus for a range of purposes. At any PC motherboard you find these:
l The floppy controller
l The serial portsl The parallel port(s)
l The keyboard controller
They all occupy IRQ's which is a central part of ISA architecture and a pain in the a... Let us take a moment
to look at these ports and controllers.
The serial ports
Serial transmission means to send data from one unit to another one bit at the time. The PC architecture
traditionally holds to RS232 serial ports. The RS-232 standard describes an asynchronous interface. This
means that data only are transmitted when the receiving unit is ready to receive them:
The serial ports are controlled by an UART chip (Universal Asynchronous Receiver Transmitter) like 16550
AFN. This chip receives bytes from the system bus and chops them up into bits. The most common
package is called 8/N/1 meaning that we send 8 bits, no parity bit and finally one stop bit. This way one byte
occupies 9 bits:
The serial ports are controlled by an UART chip (Universal Asynchronous Receiver Transmitter) like 16550
AFN. This chip receives bytes from the system bus and chops them up into bits. The most common
package is called 8/N/1 meaning that we send 8 bits, no parity bit and finally one stop bit. This way one byte
occupies 9 bits:
A Guide to Adapters and I/O units.
The serial transfer is limited to a speed of 115,200 bits per second. The cable can be up to 200 meter long.
The serial ports can be used to connect:
l The mouse
l Modems
l ISDN adapters
l Printers with serial interface
l Digital cameras
l ....
These units are connected to the serial ports using either DB9 or DB15 plugs.
In a few years time all these devices will connect to the USB bus instead.
The parallel port
Parallel transmission means that data are conducted through 8 separate wires - transmitting a full byte in
one operation. This way the parallel transmission is speedier than the serial, but the cabling is limited to
5-10 meters. The cable is fat and unhandy, holding up to 25 wires and the transmission is controlled
according to the Centronics standard.
Most printer manufactories use a 36-pins Amphenol plug, where the PC's parallel port holds a 25-pinned
connector. Hence the special printer cable. To the left you se the 25 pin connector, to the right the 36-pin:
A Guide to Adapters and I/O units.
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