-USART. Serial I/O – Programmable Communication Interface. Data Communications. Data communications refers to the ability of one computer to. USART The is a USART (Universal Synchronous Asynchronous Receiver Transmitter) for serial data communication. Interrupt Structure of . The modem control unit handles the modem handshake signals to coordinate the communication between modem and transmit control unit.

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This is a clock input signal which determines the transfer speed of transmitted data. This is the “active low” input terminal which receives a signal for reading receive data and status words from the This device also receives serial data from the outside and transmits parallel data to the CPU after conversion. As the transmitter is disabled by setting CTS “High” or command, data written before disable will be sent out.

It is possible to write a command whenever necessary after writing usrt mode instruction and sync characters. In “synchronous mode,” the baud rate is the same as the frequency of RXC. It is possible to set the status RTS by a command. In “asynchronous mode,” this is an output terminal which generates “high level”output upon the detection of a “break” character if receiver data contains a “low-level” space usagt the stop bits of two continuous characters.

CLK signal is used to generate internal device timing. Mode instruction will be in “wait for write” at either internal reset or external reset. The functional configuration is programed by software. This is a terminal whose function changes according to mode.

The device is in “mark status” high level after resetting or during a status when transmit is disabled. As a peripheral device of a microcomputer system, the receives parallel data from the CPU and transmits serial architectuge after conversion. In “synchronous mode,” the baud rate will be the same as the frequency of TXC. Command is used for setting the operation of the It is possible to see the internal status of the by reading a status word.


Intel 8251

This is the “active low” input terminal which receives a signal for writing transmit data and control words from the CPU into the The terminal will be reset, if RXD is at high level. That is, the writing of a control word after resetting will be recognized as a “mode instruction. The bit configuration of status word is shown in Fig. This is an output terminal which indicates that the is ready to accept a transmitted data character.

Table 1 shows the operation between a CPU and the device. Even if a data is written after disable, that data is not sent out and TXE will be “High”. After Reset is active, the terminal will be output at low level.

In “asynchronous mode”, it is possible to select the baud rate factor by mode instruction. If a status word is read, the terminal will be reset. Operation between the and a CPU is executed by program control. In the case of synchronous mode, it is necessary to write one-or two byte sync characters.

A “High” on this input forces the to start receiving data characters. The terminal controls data transmission if the device is set in “TX Enable” status by a command.

Intel – Wikipedia

Mode instruction is used for setting the function of the This is a terminal which indicates that the contains a character that is ready to READ. This is bidirectional data bus which receive control words and transmits data from the CPU and sends status words and received data to CPU. In “external synchronous mode, “this is an input terminal. This is an output terminal for transmitting arhcitecture from which serial-converted data is sent out.


The input status of the terminal can be recognized by the CPU reading status words. This is the “active low” input terminal which selects the at low level when the CPU accesses.

It is also possible to set the device in “break status” low level by a command. It is possible to set the status of DTR by a command.

After the transmitter is enabled, it sent out. In “synchronous mode,” the terminal is at high level, if transmit data characters are no longer remaining and sync characters are automatically transmitted. The falling edge of TXC sifts the serial data out of the Unless the CPU reads a data character before the next one is received completely, the preceding data will be lost. Data is transmitable if the terminal is at low level.


In such a case, an overrun error flag status word will be set. This is an input terminal which receives a signal archktecture selecting data or command words and status words when the is accessed by the CPU. This is an output terminal which indicates that the has transmitted all the characters and had no data character.