Before the advent of computers, the EIA (Electronic Industries Alliance) developed the RS-232 standard to connect serial devices. With the rise of personal computers, these existing serial devices became peripherals, naturally adopting the RS-232 standard. Today, the serial communication interface on PCs follows the EIA-RS-232C standard, which was last updated in 1969. This standard defines electrical characteristics, logic levels, and the functions of various signal lines.
The EIA-RS-232C standard uses positive and negative voltages to represent logic states. A voltage between -3V and -15V on a data signal line indicates logic 1, while a voltage between +3V and +15V represents logic 0. For control signals, a voltage between -3V and -15V means the signal is inactive, and between +3V and +15V means it is active. Voltages outside the range of -3V to +3V, or below -15V or above +15V, are considered invalid.
In contrast, CPUs and terminals typically use TTL (Transistor-Transistor Logic) levels with positive logic, where +5V represents logic 1 and 0V represents logic 0. Since this is incompatible with the negative logic and voltage levels used by EIA, level conversion is necessary in the interface circuit.
The EIA-RS-232C standard does not define the physical connector type, leading to the development of various connectors such as DB-25, DB-15, and DB-9. Most PC COM ports, like COM1 and COM2, use the DB-9 connector. The standard allows for a maximum cable length of 15 meters when the bit error rate is less than 4%. In practice, with a baud rate of 9600 b/s or using shielded twisted pair cables, the effective transmission distance can reach up to 30–35 meters.
The DB-9 connector for COM1 and COM2 is a male (pin) header with nine pins used for asynchronous communication. These pins include: 1 (DCD) – Data Carrier Detect, 2 (RXD) – Receive Data, 3 (TXD) – Transmit Data, 4 (DTR) – Data Terminal Ready, 5 (SG) – Signal Ground, 6 (DSR) – Data Set Ready, 7 (RTS) – Request to Send, 8 (CTS) – Clear to Send, and 9 (RI) – Ring Indicator.
The pin order for the DB-9 male connector is from left to right, while the female connector is from right to left. It’s important to pay attention to this when connecting cables. According to the EIA-RS-232C standard, there are three transmit signals: TXD, RTS, and DTR. Each line typically carries an output current of ±8mA at ±12V, providing around 192mW of power.
On the PC side, the serial port is usually male. When connecting two devices without a modem, only the receive (RXD) and transmit (TXD) pins are needed. To ensure proper communication, the RXD of one device should be connected to the TXD of the other, meaning that the signals cross over. This is why crossover cables are often used in direct device-to-device communication.
Question 1: When do you use a crossover and when do you use a direct connection?
In a standard serial port, pin 2 is RXD and pin 3 is TXD. If both devices have standard serial ports, a crossover cable is required to match the RXD and TXD pins. However, in many real-life scenarios, direct connections are used instead due to simplicity. Some development boards may have non-standard pin configurations, where pin 2 is TXD and pin 3 is RXD. In such cases, a direct cable is used when connecting to a standard serial port. Whether a serial port is standard or not depends on how the schematic is designed.
Question 2: How to identify a crossover cable and a straight-through cable?
You can use a multimeter to test connectivity. If pin 2 connects to pin 2 and pin 3 connects to pin 3, it's a straight-through cable. If pin 2 connects to pin 3 and vice versa, it's a crossover cable. If the female end is sealed, you can insert a small piece of solder wire into the socket to make contact. While testing every cable with a multimeter is time-consuming, a simple rule is: two female ends require a crossover, while one male and one female require a straight-through cable.
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