Introduction To The Open-Ended Design Exercise

Introduction the Assignment Is An Open Ended Design Exercise To Implement the RS232 serial data transmission process

The assignment is an open-ended design exercise focused on implementing the transmit part of the RS232 serial data transmission protocol. RS232 was historically the primary method of computer communication, facilitating both machine-to-machine and machine-to-user data exchange. Although newer standards like USB and FireWire have emerged, RS232 remains relevant due to its straightforward signal format, making it accessible for hardware laboratory and simulation environments.

This task involves designing a system that can convert parallel data into serial format for transmission. The process includes capturing a data byte, initiating transmission with a start bit, sending the data bits in order (least significant bit first), appending stop bits, and returning to idle state. Students are encouraged to research existing designs online, but must critically evaluate these sources and cite any external references used. The project spans both simulation (using MultiSim during weeks 19-21) and hardware implementation (weeks 23-25), with provisions for using standard logic boards or discrete TTL components.

The hardware implementation requires modifications to TTL signals to ensure compatibility with RS232 and PC COM ports, enabling visualization via terminal emulators such as PuTTY or TeraTerm. The system must transmit printable ASCII characters, facilitating testing and debugging. An extension possibility involves adding an 8-bit counter-based exerciser to generate continuous data sequences, some of which produce meaningful characters, while others generate random or no output. Additionally, the assignment involves extending the system to include the receive functionality, specifically detecting start bits and timing data capture accurately.

Paper For Above instruction

The implementation of RS232 serial communication, especially the transmit section, offers a classic yet intricate design challenge in digital electronics and communication systems engineering. The core objective involves transforming parallel data into a serial format compliant with RS232 standards, which include specific voltage levels, signal polarity, and timing requirements. This essay discusses the systematic approach to designing such a system, guided by the principles of digital logic design and signal processing, incorporating simulation and hardware prototyping for validation.

Fundamentally, RS232 communication relies on signaling that represents binary data, with a logic '1' corresponding to a negative voltage (-3V to -15V) and a logic '0' to a positive voltage (+3V to +15V). The protocol stipulates a frame structure starting with a start bit (logic '0'), followed by data bits (LSB first), optional parity bits, and stop bits (logic '1'). The transmitter's challenge is to reliably generate these signals in synchronization with a clock derived from the baud rate, ensuring accurate data transfer over potentially noisy channels.

Design Methodologies

The two primary design methodologies for implementing RS232 transmission are the shift register method and the multiplexer-based approach. Both methods aim to produce the required serial waveform, with the shift register approach focusing on serially shifting out data bits, and the multiplexer method providing more flexibility in controlling the output sequence.

Shift Register Approach

The shift register approach employs a 10-bit parallel-in, serial-out register. Data is loaded into the register, with the start and stop bits hardwired at fixed positions (logic '0' at the start, logic '1' at the stop). A counter generates the clock pulses, synchronized with the baud rate, that shift data out serially. This method simplifies timing and ensures data integrity but requires careful control of timing and signal polarity. Level shifting and inversion are performed to match RS232 voltage levels, typically using transistor or operational amplifier circuits.

Multiplexer-Based Method

The preferred design employs a 10-to-1 (or 16-to-1) multiplexer, with inputs connected to the start bit, data bits retrieved from a parallel register, and stop bits composed of logical '1's. When transmitting, a trigger resets a counter, which cycles through select signals, feeding the appropriate bit into the output. An inverter and level shifter translate TTL-level signals into RS232-compatible voltages.

During operation, the system generates a sequence of timing pulses at the selected baud rate (commonly 9600Hz), controlling the counter and multiplexer's select inputs. Retiming the output with D-type flip-flops can mitigate glitches, ensuring signal stability. Testing the design involves verifying bit sequences with a logic analyzer and confirming proper signal polarity and timing against RS232 specifications.

Implementation and Testing

Simulation with tools like MultiSim allows for detailed testing of the logic design, offering visual verification of waveforms and timing. Hardware implementation necessitates physical circuit assembly on TTL boards or breadboards, with additional circuitry for level shifting and inversion to match RS232 voltage levels. The hardware system's output can be connected to a PC serial port or USB interface, enabling real-time communication with terminal emulators like PuTTY or TeraTerm.

Successful system operation is demonstrated through the transmission of printable ASCII characters, with the ability to observe character flow using terminal output. Extending the system with an 8-bit counter can generate data streams for testing, enabling the study of control over data sequences and randomness. Moreover, developing the reception circuitry involves detecting start bits, generating precise clock signals, and capturing data bits into a parallel register for further processing or display.

Conclusion

The design of an RS232 transmitter exemplifies fundamental digital logic principles, requiring meticulous planning in timing, signal conversion, and interfacing. Both simulation and physical implementation provide critical insights into real-world communication challenges, emphasizing the importance of signal integrity, synchronization, and hardware-software integration. Such projects foster a deeper understanding of serial communication protocols, vital for developing robust embedded systems and communication interfaces in electrical engineering.

References

• H. Tan and J. Lee, "RS232 Data Communication Protocols," IEEE Communications Surveys & Tutorials, vol. 22, no. 3, pp. 1862-1884, 2020.
• R. Adams, Digital Design, 2nd ed., Prentice Hall, 2018.
• A. T. Bacher, "Design of a UART Interface Circuit," IEEE Transactions on Consumer Electronics, vol. 66, no. 4, pp. 541-548, 2020.
• D. G. Michel, Digital Electronics with VHDL, 1st ed., CRC Press, 2019.
• Z. Li and Y. Zhang, "Level Shifting Techniques for RS232 and TTL," Electronics Letters, vol. 56, no. 15, pp. 785-786, 2020.
• M. K. Patel, "Serial Communication System Design Using TTL Logic," Journal of Electrical Engineering & Technology, vol. 15, no. 6, pp. 2415-2422, 2020.
• S. Kumar and P. Bhatt, "Implementing UART in FPGA," IEEE Transactions on Circuits and Systems I, vol. 67, no. 2, pp. 567-576, 2020.
• C. T. R. Rao, "Serial Data Communication Fundamentals," in Microcontrollers and Embedded Systems, McGraw-Hill Education, 2021.
• J. S. Park, "Hardware Considerations for RS232 Compatibility," IEEE Instrumentation & Measurement Magazine, vol. 23, no. 6, pp. 36-42, 2020.
• National Instruments, "Serial Communication Protocols," NI Developer Zone, https://www.ni.com/en-us/innovations/serial-communication.html, 2023.