Uart Chips Essay, Research Paper

EET211 Introduction to Digital Electronics Lab

Fall 2000

Lab Title: Serial Communication and accessing IBM comm. ports using DOS and BIOS.

Date: 10-06-00

1. One of the main differences between serial and parallel communication is the number of wires used to transfer the data. In parallel communication, there is one line for each data bit that is being transferred where as in the case of serial communication the data is sent on a single line 1 bit at a time. The major advantage of using serial data transfers is that it is much cheaper to build the system. With only one line, any amount of data can be multiplexed and sent. A parallel transfer system would eventually have to add more lines to keep up with the increased amount of data.

2. Asynchronous communication lets all of the data move throughout the system independently which generally makes it harder to troubleshoot or design. A common clock that goes to all parts of the system clocks a synchronous communication system. Operations can only be performed within the system or circuit on each clock pulse. This will synchronize all of the communications within the circuit. Because everything happens at the same time, it is easier to predict the outputs of any component and therefore easier to troubleshoot and design than an asynchronous system.

3. There are three basic types of communication, simplex, half-duplex, and full-duplex. Simplex communication means that the communication can only go one direction such as a computer to a printer. There is no need for a printer to send data back to the computer so a simplex communication system could be used. Half-duplex communication systems support data transfer in both directions but not at the same time where as a full-duplex system supports data transfer in both directions simultaneously. A half-duplex system has the advantage of only having one data transfer line that needs to be run but the disadvantage is that one device must wait until the other is done to transmit. A full-duplex system does have the ideal case in which data is transferred simultaneously in both directions but the disadvantage of this is that an extra data line must be run between the devices which may prove more costly than the electronic switching hardware that would be necessary to use half-duplex.

4. RS-232 was introduced in 1960, and is currently the most widely used communication standard. It is simple, inexpensive to implement, and though relatively slow, it is more than adequate for most simple serial communication devices such as keyboards and mice. RS-232 is a serial data transmission system, which means that it uses a single wire for data transmission. Signals are processed by determining whether they are positive or negative when compared with a ground. RS-232 systems are recommended for communication over short distances (up to 50 feet) and at relatively slow data rates, (up to 20 kbps). Sometimes these limits can be exceeded for certain applications but these are the basic standards. RS-232 also defines the meaning of the different serial signals and their respective pin assignments on a standard 25-pin (DB-25) serial connector. These assignments are shown in the attached table.

5. UART is an acronym that stands for Universal Asynchronous Receiver/Transmitter. The UART chip is a computer component that handles asynchronous serial communications and manages all serial ports. All internal modems also contain a UART. Because the receiver and transmitter are independent in the chip, the UART can implement a full-duplex system. Although the UART is an asynchronous device, it still requires a clock signal to determine the transmission rate.

6. When an asynchronous system is idle, the output is a high or a “1”. The start bit is the first bit that is sent and it is a low or “0”. This indicates that the next several bits (whatever the system is designed for) are going to be data bits. The data bits are transferred starting with the LSB and proceeding in order to the MSB or parity bit. A stop bit is a “1” that is sent after the data bits to indicate the end of the transmission. This “1” resets the signal to the idle state so it is ready to wait or transfer more data. For example, if we wanted to represent the ASCII character ‘A’ it would normally be represented by the binary number 01000001. If we wanted to transfer this letter ‘A’ across and asynchronous serial transmission system we would have to include the start and stop bits and the new representation would be 0010000011.

7. An interrupt function allows the assembly programmer to call up an operating system subroutine. This function is denoted by the command INT. The 14 in the command INT14H refers to the interrupt that will initialize the asynchronous communications port and return the status of that port.

DB-25 pin assignments

Pin Name Signal Pin Name Signal

1 FG Frame ground 14 STD Secondary TD

2 TD Transmitted data 15 TC Transmit clock

3 RD Received data 16 SRD Secondary RD

4 RTS Request to send 17 RC Receive clock

5 CTS Clear to send 18 – Unassigned

6 DSR Data set ready 19 SRTS Secondary RTS

7 SG Signal ground 20 DTR Data terminal ready

8 DCD Data carrier signal 21 SQ Signal quality detector

9 – Positive voltage 22 RI Ring indicator

10 – Negative voltage 23 DRS Data rate selector

11 – Unassigned 24 SCTE Clock transmit external

12 SDCD Secondary DCD 25 BUSY Busy

13 SCTS Secondary CTS

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