Before the advent of computers, serial devices needed a standard to communicate, so EIA (Electronic Industries Alliance) developed the RS-232 standard. Once personal computers emerged, these existing serial devices became peripherals and naturally adopted 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 uses positive and negative voltages to represent logic states. A voltage between -3V and -15V on a data 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's active. Voltages outside this range (-3V to +3V or below -15V and above +15V) are considered invalid.
In contrast, CPUs and terminals typically use TTL (Transistor-Transistor Logic) levels, which operate with +5V for logic 1 and 0V for logic 0. These levels are not compatible with the negative logic used by EIA, so level conversion is necessary in the interface circuit. The EIA-RS-232C does not specify the physical connector type, leading to the development of different connectors such as DB-25, DB-15, and DB-9. Most PC COM ports, like COM1 and COM2, use the DB-9 connector.
According to the EIA-RS-232C standard, when the bit error rate is less than 4%, the maximum cable length is 15 meters. In practice, with a baud rate of 9600 b/s or using shielded twisted pair cables, the transmission distance can extend up to 30 to 35 meters.
The DB-9 connector has nine pins, each with a specific function. Pin 1 is Data Carrier Detect (DCD), pin 2 is Receive Data (RXD), pin 3 is Transmit Data (TXD), pin 4 is Data Terminal Ready (DTR), pin 5 is Signal Ground (SG), pin 6 is Data Set Ready (DSR), pin 7 is Request to Send (RTS), pin 8 is Clear to Send (CTS), and pin 9 is Ring Indicator (RI). The male DB-9 connector has pins arranged from left to right, while the female connector has them from right to left.
The EIA-RS-232C standard defines three transmit signals: TXD (transmit data), RTS (request to send), and DTR (data terminal ready). Each line typically carries ±8mA at ±12V, providing about 192mW of power.
On the PC side, the serial port is usually a male connector. When connecting two devices without a modem, only the RXD (pin 2) and TXD (pin 3) lines are needed. One device’s RXD should be connected to the other’s TXD, meaning the connections are crossed. However, in many real-world cases, direct connections are used instead of crossover cables due to simplicity.
To determine if a serial cable is a crossover or straight-through, you can use a multimeter. If pin 2 connects to pin 2 and pin 3 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 testing a female connector is difficult, you can use a small piece of solder wire to make contact with the internal metal parts. Alternatively, remember that a cable with two female ends is a crossover, while one with a male and a female end is a straight-through cable.
Understanding whether a serial port is standard or non-standard is essential. On some development boards, the RXD and TXD pins may be swapped to allow direct connection with a standard serial port. Always check the schematic to confirm the configuration. In general, RS-232 DB-9 cables come in three types: male-to-female, male-to-male, and female-to-female, and each can be either straight or crossover, resulting in six possible configurations.
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