-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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The terminal will be reset, if Usarh is at high level. Table 1 shows the operation between a CPU and the device. The device is in “mark status” high level after resetting or during a status when transmit is disabled.

In the case of architectjre mode, it is necessary to write one-or two byte sync characters. As the transmitter is disabled by setting CTS “High” or command, data written before disable will be sent out.

In “internal synchronous mode. In “synchronous mode,” the baud rate is the same as the frequency of RXC. A “High” on this input forces the into “reset status.

After the transmitter is enabled, it sent out.

Mode instruction is used for setting the function of the A “High” on this input forces the to start receiving data characters. It is also possible to set the device in “break status” low level by a command.

In “synchronous mode,” arcyitecture terminal is at high level, if transmit data characters are no longer remaining and sync characters are automatically transmitted. This is an architecturr terminal for transmitting data from which serial-converted data is sent out.


The input status arfhitecture the terminal can be recognized by the CPU reading status words. In such a case, an overrun error flag status word will be set. In “asynchronous mode,” it is possible to select the baud rate factor by mode instruction. This device also receives serial data from the outside and transmits parallel data to the CPU after conversion. After Atchitecture is active, the terminal will be output at low level.

That is, the writing of a control word after resetting will be recognized as a “mode instruction. Even if a data is written after disable, that data is not sent out and TXE will be “High”. Unless the CPU reads a data character before the next one is received completely, the preceding data will be lost. It is possible to set the status of DTR by a command. This is a clock input signal which determines the transfer speed of received data. The falling edge of TXC sifts the serial data out of the Data is transmitable if the terminal is at low level.

This is an output terminal which indicates that the is ready to accept a transmitted data character.

Intel – Wikipedia

The bit configuration of status word is shown in Fig. In “external synchronous mode, “this is an input terminal. CLK signal is used to generate internal device timing. This is bidirectional data bus which receive control words and transmits data from the CPU and sends status words and received data to CPU. This is the “active low” input terminal which receives a signal for writing transmit data and control words from the CPU into the This is a terminal whose function changes according to mode.


It is possible to write a command whenever necessary after writing a mode instruction and sync characters. This is a clock input signal which determines the transfer speed of transmitted data. The functional configuration is programed by software. If a status word is read, the terminal will be reset. In “asynchronous mode”, it is possible to select the baud rate factor by mode instruction.

If sync characters were written, a function will be set because the writing of sync characters constitutes part of mode instruction.

The bit configuration of mode instruction is shown in Figures 2 and 3. This is the “active low” input terminal which selects the at low level when the CPU accesses.

Intel 8251

This is a terminal acrhitecture indicates that the contains a character that is ready to READ. This is an input terminal which receives a signal for selecting data or command words and status words when the is accessed by the CPU.

It is possible to set the status RTS by a command.

As a peripheral device of a microcomputer system, the receives parallel data from the CPU and transmits serial data after conversion. The terminal controls data transmission if the device is set in “TX Enable” status by a command. Operation between the and a CPU is executed by program control.