RS-232 Protocol

RS-232 is a standard protocol used for serial communication, widely utilized in industrial automation for connecting computers, controllers, and other devices. Understanding RS-232 protocol and related calculations is essential for optimizing communication in automation systems.

Overview of RS-232 Protocol

RS-232 (Recommended Standard 232) defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors. It is primarily used for short-distance communication.

Key Features of RS-232

• Asynchronous Communication: Data is transmitted one byte at a time without a clock signal.
• Voltage Levels: Uses ±12V signals, with -12V to -3V representing a logic 1 (mark) and +3V to +12V representing a logic 0 (space).
• Connectors: Commonly uses DB-9 or DB-25 connectors.
• Baud Rate: The speed of data transmission, typically ranging from 300 to 115200 bits per second (bps).

RS-232 Signal Lines

RS-232 defines several signal lines for data transmission and control:

• TD (Transmit Data): Carries data from the DTE (Data Terminal Equipment) to the DCE (Data Communication Equipment).
• RD (Receive Data): Carries data from the DCE to the DTE.
• RTS (Request to Send): Indicates that the DTE is ready to send data.
• CTS (Clear to Send): Indicates that the DCE is ready to receive data.
• DSR (Data Set Ready): Indicates that the DCE is ready to establish a link.
• DTR (Data Terminal Ready): Indicates that the DTE is ready to establish a link.
• GND (Ground): Common reference voltage.

Baud Rate Calculation

The baud rate is the speed at which data is transmitted over the RS-232 interface. It is measured in bits per second (bps). To ensure proper communication, both devices must be set to the same baud rate.

Example Calculation

Determine the time it takes to transmit a single byte (8 bits) at a baud rate of 9600 bps.

• Task: Calculate the transmission time for one byte.
• Solution: Use the formula Transmission Time = Number of Bits / Baud Rate.
• • Transmission Time = 8 bits / 9600 bps ≈ 0.000833 seconds
  • Transmission Time ≈ 833 microseconds

RS-232 Data Framing

RS-232 uses data framing to organize the bits being transmitted. A typical frame includes:

• Start Bit: Indicates the beginning of a data frame.
• Data Bits: The actual data being transmitted (typically 7 or 8 bits).
• Parity Bit: Optional bit used for error checking (even, odd, or none).
• Stop Bit: Indicates the end of a data frame (typically 1 or 2 bits).

RS-232 Connectors, Cables, and Pinout

Types of RS-232 Connectors

RS-232 interfaces commonly use two types of connectors:

1. DB-9 Connector

The DB-9 connector is the most common connector used for RS-232 communication. It has 9 pins and is often found on PCs and other equipment.

Pinout:

• Pin 1: DCD (Data Carrier Detect)
• Pin 2: RD (Receive Data)
• Pin 3: TD (Transmit Data)
• Pin 4: DTR (Data Terminal Ready)
• Pin 5: GND (Signal Ground)
• Pin 6: DSR (Data Set Ready)
• Pin 7: RTS (Request to Send)
• Pin 8: CTS (Clear to Send)
• Pin 9: RI (Ring Indicator)

2. DB-25 Connector

The DB-25 connector is less common than the DB-9 but is still used in some applications. It has 25 pins and provides additional signal lines.

Pinout:

• Pin 1: GND (Shield Ground)
• Pin 2: TD (Transmit Data)
• Pin 3: RD (Receive Data)
• Pin 4: RTS (Request to Send)
• Pin 5: CTS (Clear to Send)
• Pin 6: DSR (Data Set Ready)
• Pin 7: GND (Signal Ground)
• Pin 8: DCD (Data Carrier Detect)
• Pin 9: +Voltage (Not Used)
• Pin 10: -Voltage (Not Used)
• Pin 11: Unassigned
• Pin 12: Secondary RD
• Pin 13: Secondary CTS
• Pin 14: Secondary TD
• Pin 15: Transmitter Clock
• Pin 16: Secondary DCD
• Pin 17: Receiver Clock
• Pin 18: Unassigned
• Pin 19: Secondary RTS
• Pin 20: DTR (Data Terminal Ready)
• Pin 21: Signal Quality Detector
• Pin 22: Ring Indicator
• Pin 23: Data Signal Rate Selector
• Pin 24: Transmitter Clock (External)
• Pin 25: Unassigned

Types of RS-232 Cables

RS-232 cables can be categorized based on their wiring configuration:

1. Straight-Through Cable

In straight-through cables, the pin connections are one-to-one. This means Pin 1 on one end is connected to Pin 1 on the other end, Pin 2 to Pin 2, and so on. These cables are typically used to connect DTE (Data Terminal Equipment) to DCE (Data Communication Equipment).

2. Null Modem Cable

Null modem cables are used for direct communication between two DTEs without a DCE. In this configuration, the transmit and receive lines are crossed. For example, Pin 2 (TD) on one end is connected to Pin 3 (RD) on the other end, and vice versa.

RS-232 Pinout

The pinout configuration for DB-9 and DB-25 connectors is as follows:

DB-9 Pinout

• Pin 1: DCD (Data Carrier Detect)
• Pin 2: RD (Receive Data)
• Pin 3: TD (Transmit Data)
• Pin 4: DTR (Data Terminal Ready)
• Pin 5: GND (Signal Ground)
• Pin 6: DSR (Data Set Ready)
• Pin 7: RTS (Request to Send)
• Pin 8: CTS (Clear to Send)
• Pin 9: RI (Ring Indicator)

DB-25 Pinout

• Pin 1: GND (Shield Ground)
• Pin 2: TD (Transmit Data)
• Pin 3: RD (Receive Data)
• Pin 4: RTS (Request to Send)
• Pin 5: CTS (Clear to Send)
• Pin 6: DSR (Data Set Ready)
• Pin 7: GND (Signal Ground)
• Pin 8: DCD (Data Carrier Detect)
• Pin 9: +Voltage (Not Used)
• Pin 10: -Voltage (Not Used)
• Pin 11: Unassigned
• Pin 12: Secondary RD
• Pin 13: Secondary CTS
• Pin 14: Secondary TD
• Pin 15: Transmitter Clock
• Pin 16: Secondary DCD
• Pin 17: Receiver Clock
• Pin 18: Unassigned
• Pin 19: Secondary RTS
• Pin 20: DTR (Data Terminal Ready)
• Pin 21: Signal Quality Detector
• Pin 22: Ring Indicator
• Pin 23: Data Signal Rate Selector
• Pin 24: Transmitter Clock (External)
• Pin 25: Unassigned