Parallel Interface

The parallel interface, also known as the parallel port, uses a 25-pin D-type connector. The term "parallel" means that it can transmit 8 bits of data simultaneously via parallel lines, which significantly boosts the data transfer speed. However, the length of parallel transmission lines is limited. As the length increases, interference rises and data is more prone to errors. Currently, the parallel interface is mainly used as a printer port, etc.

Working Modes of the Parallel Port

• SPP, short for Standard Parallel Port, is the first parallel port working mode. Almost all peripheral devices using parallel ports support this mode.

• EPP, or Enhanced Parallel Port, is a new working mode developed based on SPP. It is also the most widely used parallel port working mode. Currently, most printers and scanners on the market support the EPP mode.

• ECP, which stands for Extended Capability Port, is a relatively advanced parallel port working mode but has more compatibility issues. Do not select this mode unless the peripheral devices support ECP.

Serial Port

The serial port, also known as the serial communication interface or COM port, includes RS-232-C, RS-422, RS-485, and USB according to electrical standards and protocols. The RS-232-C, RS-422, and RS-485 standards only specify the electrical characteristics of the interface and do not cover connectors, cables, or protocols. USB is a new interface standard developed in recent years, mainly used in the field of high-speed data transfer.

1. RS-232-C

RS-232-C, also known as the standard serial port, is the most commonly used serial communication interface at present. It is a serial communication standard formulated by the Electronic Industries Alliance (EIA) of the United States, Bell System companies, modem manufacturers, and computer terminal manufacturers in 1970. Its full name is "Interface Standard for the Serial Binary Data Exchange between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)".

The traditional RS-232-C interface standard has 22 wires and uses a standard 25-pin D-type plug and socket. Later PCs used a simplified 9-pin D-type socket. The 25-pin plug and socket are rarely used in applications nowadays. Today's computers usually have two serial ports, COM1 and COM2. You can see the 9-pin D-type interface on the back of the computer. Nowadays, many mobile phone data cables or logistics receivers are connected to the computer via the COM port.

2. RS-422

To address the shortcomings of short communication distance and low speed of RS-232, RS-422 defines a balanced communication interface, increasing the transmission rate to 10Mb/s and the transmission distance to 4,000 feet (when the rate is greater than 100kb/s). It can connect up to 10 receivers on a balanced bus. RS-422 is a one-way balanced transmission specification for single-transmitter and multi-receiver applications, known as the TIA/EIA-422-A standard.

3. RS-485

To expand the application range, the EIA formulated the RS-485 standard based on RS-422 in 1983, adding multi-point and two-way communication functions, that is, allowing multiple transmitters to be connected to the same bus.

The driving ability and collision protection function of the transmitters have been enhanced, and the common-mode range of the bus has been expanded. It is later known as the TIA/EIA-485-A standard.

4. USB

Universal Serial Bus (USB) is a new peripheral device interface standard initiated by several major manufacturers such as Intel, Microsoft, Compaq, IBM, NEC, and Northern Telcom. The USB interface is a four-pin interface on the computer motherboard. The two middle pins transmit data, and the two pins on both sides supply power to peripheral devices. The USB interface features high speed, simple connection, and no need for an external power supply. Its transmission speed is 12Mbps, and the latest USB 2.0 can reach 480Mbps.

The maximum length of the cable is 5 meters. A USB cable has 4 wires, 2 signal wires, and 2 power wires, providing 5 volts of power. USB cables are classified into shielded and unshielded types. The shielded cable can achieve a transmission speed of 12Mbps at a higher price, while the unshielded cable has a speed of 1.5Mbps but is cheaper. USB can connect up to 127 devices in series and supports the hot-plug function.

5. RJ45

The RJ45 interface is the most commonly used interface in Ethernet. RJ45 is a general term, referring to the modules standardized by IEC (60)603-7, and modified jacks or plugs defined by the international connector standard with 8 positions (8 pins).

6. The Serial Port of the Switch

The serial port of the switch, known as "trunk" in English, is used for the next-hop routing conversion. Each VLAN can be connected to the external network only after being routed to the trunk.

Differences Between Serial and Parallel Ports

The serial port is described as a single channel, while the parallel port has 8 channels that can transmit 8 bits (one byte) of data simultaneously. However, due to the mutual interference among the 8-bit channels, it is not as fast as expected in parallel transmission. The speed is limited during transmission.

Moreover, when a transmission error occurs, all 8 bits of data must be retransmitted simultaneously. The serial port has no such interference; only one bit needs to be resent in case of a transmission error. Therefore, it is faster than parallel processing. Serial hard drives are valued in this way.