Serial I/O Lines in 8085 Microprocessor

Introduction :

The 8085 microprocessor is a widely used 8-bit microprocessor that was developed by Intel. One of the important features of the 8085 microprocessor is its ability to support serial input/output (I/O) operations. The 8085 microprocessor has two dedicated pins, namely, the Serial Data Input (SID) and the Serial Data Output (SOD), that are used for serial communication.

The SID pin is used to input data serially into the microprocessor, while the SOD pin is used to output data serially from the microprocessor. The SID and SOD pins are controlled by the Serial Control (SOD) pin, which is used to indicate the start and end of a data transfer operation.

The serial communication in the 8085 microprocessor is based on the asynchronous serial communication protocol, which is also known as the start-stop communication protocol. In this protocol, data is transmitted as a sequence of bits, where each bit is preceded by a start bit and followed by one or more stop bits.

The 8085 microprocessor also supports serial communication using an interrupt-driven approach. In this approach, the microprocessor is interrupted when a data transfer operation is initiated, and it then handles the data transfer operation using interrupt service routines.

why use Serial I/O Lines in 8085 Microprocessor ?

Serial input/output (I/O) lines are used in the 8085 microprocessor for various reasons, including:

1. Cost-effectiveness: Serial communication requires fewer wires than parallel communication, which makes it a more cost-effective option for transferring data in many applications.
2. Compatibility: Many devices and components, such as sensors, displays, and memory chips, use serial communication protocols. The use of serial I/O lines in the 8085 microprocessor makes it compatible with these devices and components.
3. Data transfer over longer distances: Serial communication is more reliable than parallel communication for data transfer over longer distances. This is because parallel communication signals can be degraded due to the effects of noise and signal attenuation over longer distances.
4. Simplified data transfer: Serial communication involves sending data one bit at a time, which can simplify the process of data transfer and reduce the complexity of the hardware required for interfacing with other devices.
5. Flexibility: Serial communication allows for greater flexibility in terms of the baud rate, data format, and other communication parameters. This makes it easier to adjust the communication protocol to meet the requirements of different devices and applications.

The 8085 Microprocessor has Serial Input/Output lines consisting of two pins as follows:

1. Serial Output Data (SOD)
2. Serial Input Data (SID) 

They both are specially made for Input/Output which is further controlled by software. The transfer of data is controlled with the help of two instructions, i.e, SIM and RIM. Now, let’s see both the pins one-by-one:

1. Serial Output Data (SOD): The SIM Instruction should be initiated in order to output data in serial manner. This is done through the SOD Line. Examples:

Instructions: MVI A, 80H : Set D7 in the accumulator=1 RAR : Set D6 = 1 and bring carry into D7 SIM : Output D7

1. In the above set of instructions, the serial output line is enabled by rotating 1 into bit position D6; the instruction SIM outputs the carry bit through bit position D7. It can be represented as: Figure – Interpretation by SIM Instruction
2. Serial Input Data (SID): In SID, the RIM Instruction is initiated to input data in a serial manner. This is done through SID line. It can be represented as: Figure – Interpretation by RIM Instruction

Advantage: In a software controlled Input/Output system, the SID and SOD lines eliminate the need of an input and output port respectively.

Issues of Serial I/O Lines in 8085 Microprocessor :

 Here are some of the common issues with serial I/O lines in the 8085 microprocessor:

1. Limited data transfer rates: Serial communication typically has slower data transfer rates than parallel communication, which may limit the performance of the system in certain applications.
2. Data transmission errors: Serial communication is more susceptible to data transmission errors than parallel communication, particularly in the presence of noise or interference in the communication channel. This may result in corrupted or lost data.
3. Complex synchronization: Asynchronous serial communication, which is commonly used in the 8085 microprocessor, requires complex synchronization mechanisms to ensure that the data is transferred correctly. This may add complexity to the design and implementation of the system.
4. Interrupt handling: The interrupt-driven approach to serial communication in the 8085 microprocessor requires careful handling of interrupts to avoid data loss or corruption.
5. Hardware limitations: The 8085 microprocessor has a limited number of pins, which may restrict the number of devices that can be connected to the system using serial communication.

Uses of Serial I/O Lines in 8085 Microprocessor :

Some of the common uses of serial I/O lines in the 8085 microprocessor are:

1. Interfacing with sensors and actuators: Many sensors and actuators use serial communication protocols for data transfer, making serial I/O lines a popular choice for interfacing with these devices. This allows the 8085 microprocessor to monitor and control various parameters in real-time.
2. Data logging and storage: Serial communication can be used to transfer data to and from storage devices such as memory chips, allowing the 8085 microprocessor to log and store data over time.
3. Industrial automation: Serial I/O lines are commonly used in industrial automation systems, where they provide a cost-effective and reliable way to transfer data between different components of the system.
4. Communication with peripherals: Serial communication can be used to communicate with peripherals such as printers, displays, and modems, allowing the 8085 microprocessor to interact with these devices.
5. Networking: Serial communication can be used to establish networking connections between different devices, allowing the 8085 microprocessor to transfer data over a network.