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Unformatted text preview: CSCI 120 Introduction to Computation Ports and I/O devices (draft) Saad Mneimneh Visiting Professor Hunter College of CUNY 1 Ports As we have seen in previous lectures, communication between the computer (CPU and memory) and I/O devices such as disks, keyboards, mice, printers, monitors, etc... is normally handled through an intermediate device known as a controller. The controller connects to the I/O device by cables either within the computer case, or to a connector on the back (or side) of the computer case, known as port . The I/O device is then attached to the port externally by a corresponding connector. memory CPU I/O I/O devices computer I/O I/O motherboard serial and parallel ports expansion slots or cables Figure 1: I/O ports Often, the words bus, port, and connector are used interchangeably. There are different kinds of ports: 1.1 Serial ports A serial port connects the I/O device to the I/O controller where transfer of data is performed one bit at a time. Therefore, the original use of serial ports was usually to connect devices that do not require fast data transmission rates, such as mouse, modem ( mo dulation demo dulation device for connecting the computer to the phone line), network by allowing two computers to connect together and exchange data, and printers. The serial port is also referred to as COM port (short for communication) because it was mainly intended to interface with a modem or a similar communication device. Serial ports conform to either the RS-232 standard (25 pins) or the RS-422 standard (9 pins as shown in Figure 2). 1 5 6 9 Figure 2: Serial port The 9 pins (signals): 1. carrier detect: determines if the modem is connected to a working phone line 2. receive data: computer receives information sent from the modem 3. transmit data: computer sends information to the modem 4. data terminal ready: computer tells the modem that it is ready to talk 5. signal ground: pin is grounded (low voltage) 6. data set ready: modem tells the computer that it is ready to talk 7. request to send: computer asks the modem if it can send information 8. clear to send: modem tells the computer that it can send information 9. ring indicator: once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected RS-422 uses twisted pair wires for each signal allowing more effective com- munication (faster transmission rate over longer lines). The reason why such communication can be achieved is that twisted pairs reduce the effect of cross- talk and noise: Cross-talk (signals leaking between wires in a cable) is minimized: Signals are sent down twisted pair wires such that when one wire in the pair becomes positive the other wire becomes negative by the same amount....
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- Spring '09