SILICON GATE MOS 8251

Modem Control

The 8251 has a set of control inputs and outputs that can be used to simplify the interface to almost any Modem. The modem control signals are general purpose in nature and can be used for functions other than Modem control, if necessary.

DSR (Data Set Ready)

The DSR input signal is general purpose in nature. Its con- dition can be tested by the CPU using a Status Read opera- tion. The DSR input is normally used to test Modem con- ditions such as Data Set Ready.

TxE (Transmitter Empty)

When the 8251 has no characters to transm it, the T x E out- put will go "high". It resets automatically upon receiving a character from the CPU. TxE can be used to indicate the end of a transmission mode, so that the CPU "knows" when to "turn the line around" in the half-duplexed operational mode.

In SYNChronous mode, a "high" on this output indicates that a character has not been loaded and the SYNC charac- ter or characters are about to be transmitted automatically as "fillers".

DTR (Data Termin·al Ready)

The DTR output signal is general purpose in nature. It can be set "low" by programming the appropriate bit in the Command Instruction word. The DTR output signal is norm- ally used for Modem control such as Data Terminal Ready or Rate Select.

RTS (Request to Send)

The RTS output signal is general purpose in nature. It can be set "low" by programr:ning the appropriate bit in the Command Instruction word. The RTS output signal is norm- ally used for Modem control such as Request to Send.

CTS (Clear to Send)

A "low" on this input enables the 8251 to transmit data (serial) if the Tx EN bit in the Command byte is set to a

" one."

Transmitter Buffer

The Transmitter Buffer accepts parallel data from the Data Bus Buffer, converts it to a serial bit stream, inserts the ap- propriate characters or bits (based on the 'communication technique) and outputs a composite serial stream of data on the TxD output pin.

Transmitter Control

The Transmitter Control manages all activities associated with the transmission of serial data. It accepts and issues signals both externally and internally to accomplish this function.

TxRDY (Transmitter Ready)

This output signals the CPU that the transmitter is ready to accept a data character~ It can be used as an interrupt to the system or for the Polled operation the CPU can check TxRDY using a status read operation. TxRDY is automatic- ally reset when a character is loaded from the CPU.

TxC (Transmitter Clock)

The Transmitter Clock controls the rate at which the char- acter is to be transmitted. In the Synchronous transmission mode, the frequency of TxC is equal to the actual Baud Rate (1 X). In Asynchronous transmission mode, the fre- quency of TxC is a multiple of the actual Baud Rate. A portion of the mode instruction selects the value of the multiplier; it can be 1x, 16x or 64x the Baud Rate.

For Example:

If Baud Rate equals 110 Baud,

TxC equals 110 Hz (1x)

TxC equals 1.76 kHz (16x)

TxC equals 7.04 kHz (64x).

If Baud Rate equals 9600 Baud,

TxC equals 614.4 kHz (64x).

The falling edge of TxC shifts the serial data out of the 8251.

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Intel 8080 manual Modem Control, DSR Data Set Ready, TxE Transmitter Empty, DTR Data Termin·al Ready, RTS Request to Send
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8080 specifications

The Intel 8085 and 8080 microprocessors were groundbreaking innovations in the world of computing, paving the way for future microprocessor development and personal computing.

The Intel 8080, introduced in 1974, was an 8-bit microprocessor that played a fundamental role in the early days of personal computing. With a 16-bit address bus, it had the capability to address 64 KB of memory. Running at clock speeds of 2 MHz, the 8080 was notable for its instruction set, which included 78 instructions and 246 opcodes. It supported a range of addressing modes including direct, indirect, and register addressing. The 8080 was compatible with a variety of peripherals and played a crucial role in the development of many early computers.

The microprocessor's architecture was based on a simple and efficient design, making it accessible for hobbyists and engineers alike. It included an 8-bit accumulator, which allowed for data manipulation and storage during processing. Additionally, the 8080 featured registers like the program counter and stack pointer, which facilitated program flow control and data management. Its ability to handle interrupts also made it suitable for multitasking applications.

The Intel 8085, introduced in 1976, was an enhancement of the 8080 microprocessor. It maintained a similar architecture but included several key improvements. Notably, the 8085 had a built-in clock oscillator, simplifying system design by eliminating the need for external clock circuitry. It also featured a 5-bit control signal for status line management, which allowed for more flexible interfacing with peripheral devices. The 8085 was capable of running at speeds of up to 3 MHz and had an extended instruction set with 74 instructions.

One of the standout features of the 8085 was its support for 5 extra instructions for stack manipulation and I/O operations, which optimized the programming process. Additionally, it supported serial communication, making it suitable for interfacing with external devices. Its 16-bit address bus retained the 64 KB memory addressing capability of its predecessor.

Both the 8080 and 8085 microprocessors laid the groundwork for more advanced microprocessors in the years that followed. They demonstrated the potential of integrated circuits in computing and influenced the design and architecture of subsequent Intel microprocessors. Their legacy endures in the way they revolutionized computing, making technology accessible to a broader audience, and their influence is still felt in the design and architecture of modern microprocessors today.
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