Interfacing RS-232 and TTL/CMOS Devices: A Comprehensive Guide
When interfacing devices using RS-232 and TTL/CMOS logic levels, it is crucial to understand the nuances of these communication standards. This article aims to provide a detailed explanation of potential issues and solutions when transitioning between these two technologies, particularly when interfacing 12V and 5V devices.
Understanding RS-232 and TTL/CMOS Logic Levels
RS-232 is a serial communication standard that operates with negative voltages to represent logic levels. Specifically, in the -12V RS-232 standard, a logical 1 is represented by a negative voltage, while a logical 0 is represented by a positive voltage. On the other hand, TTL and CMOS standards are compatible with CMOS logic levels, where a low voltage (0.7V) represents a logical 0 and a high voltage (2.4V) represents a logical 1.
The UART Communication Protocol
The Universal Asynchronous Receiver-Transmitter (UART) is a standard for asynchronous serial communication. It uses a start bit, 8 data bits (LSB-first), and a stop bit. The communication protocol also employs idle states, where the UART transmits a 1 while not transmitting data.
Common Issues and Solutions
In the context of interfacing 12V and 5V devices, a common issue arises due to the reversal of logic levels, which can lead to framing errors. For example, if a device on the 12V side sends 41, it sets the first bit as a logical 1 (negative voltage), while the 5V side interprets this as a logical 0. This can cause the UART on the 5V side to misinterpret the start and stop bits, leading to framing errors.
Example with 41 on RS-232 and 5V Levels
Let's break down the process step-by-step:
1. Sent from 12V RS-232 side: The RS-232 sender sends a start bit of 0, followed by 8 data bits (LSB-first), then a stop bit of 1. The signal for 41 would be sent as 1 (negative voltage) followed by 8 data bits and a trailing 1.
2. Received at 5V side: The TTL-compatible UART receiver interprets the received signal. The first 1 is incorrectly read as a Stop bit and the next 0 as a Start bit. The receiver then decodes the next 8 bits, leading to incorrect data. This process will repeat, causing frequent framing errors.
Solutions and Recommendations
To resolve these issues, it is essential to use an inverting level shifter, such as the MAX232, to adapt between the RS-232 levels and TTL/CMOS levels. The MAX232 device performs both inversion and level shifting, ensuring that the signals are correctly interpreted by the receiving device. While the MAX232 is a well-established and widely-used solution, there are newer and more sophisticated options available in the market that might provide better performance and lower power consumption.
Additional Considerations
According to Clem Cole, the actual RS-232 signal levels for mark and space are even wider than expected: -30V to -3V for mark and 3V to 30V for space. Furthermore, older RS-232 devices often operated with a -9V to 9V voltage swing. This additional information emphasizes the importance of using robust level-shifting solutions and verifying the voltage levels in your application.
Conclusion
In summary, when interfacing devices with different logic levels, such as RS-232 and TTL/CMOS, it is crucial to understand the nuances of each standard. Using inverting level shifters, such as the MAX232, is a reliable solution to ensure correct signal interpretation and avoid common issues like framing errors. Always consider the specific voltage levels and requirements of your devices to guarantee successful communication.