The Difference Between Serial vs. Parallel Communication

LAVA has been designing and manufacturing custom serial interfaces for 36 years, and in that time we established ourselves as industry experts on serial and parallel communication. While technological advancements have largely transitioned to fast communication standards, serial and parallel still play an important role at LAVA and beyond. This article will provide an overview on what Serial and Parallel communication are, the differences between them, and how their advantages translate into different use applications.

Serial communication and transmission is the process of sending data sequentially, one bit at a time over a communications medium. In the most classic sense, Morse code could be considered a version of serial communication. But for modern purposes serial communication equipment is used to transmit data with cables.

The type of communication equipment that you’re planning to connect to a serial port will dictate the type of cabling required to successfully communicate with the hardware. There are two types of serial communication equipment: Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). For most practical purposes a DTE would refer to the serial port on the back of a PC and/or a standard POS device (i.e. a barcode scanner), while a DCE would refer to something like a modem. When connecting a DTE device to a DCE, a standard straight through cable is required. But when connecting a DTE to another DTE, then a null model, or crossover cable is required to connect the ports.

Once a serial device has been connected to a serial port, data is transmitted sequentially over the Tx (transmit) and Rx (receive) lines located on the serial port, with 1s and 0s being measured in reference to the GND (Ground) line. The other lines on a serial port can handle hardware flow control but are not always required. Each block of data is transmitted at a predetermined speed (i.e. baud rate) so in order for both ends to communicate correctly, both the sender and receiver must know the rate that data is being sent, as well as how the byte of data is formed. The maximum baud rate for a standard RS-232 serial port is 115kpbs. Depending on how the serial port is configured, data can be transmitted in one of several different formats. For example 8-N-1 would be used to transmit data in 8 bit segments with No Parity and 1 Stop Bit. The most common standard (RS-232) has a distance limitation of only 15m (or 50ft), but this can be extended by using different cabling mediums (i.e. cat-5) or lower baud rates.

Serial communication is a reliable standard that has been used for decades because it is easy to implement, interface with, and perfect for monitoring devices where data may be occasional and periodic. Despite its age, a surprising number of electronic devices are still equipped with a simple UART for debugging. For reference, LAVA serial cards are typically used in Point of Sale (POS) environments (i.e. cash registers, barcode scanners and receipt printers) and for modem and polling communications. Yet they also find applications in manufacturing/industrial settings such as CNC machines and monitoring devices such as weight scales and temperature sensors.

Parallel communication and transmission is the process of sending multiple binary digits simultaneously. It requires multiple lines for transmission as opposed to the single wire required for serial communication. Connecting parallel devices requires specific cables to account for the multiple transmission lines. The most common parallel cable is the IEEE 1284-B (i.e. Centronics) Cable which is used to connect a PC to a parallel printer. Another is the IEEE 1284-A (i.e. D-sub) Cable which is less common, but used for connecting a PC to a non-printer peripheral that communicate over parallel (i.e. older Zip Drives). There is no standard maximum cable length for parallel communication, but generally the cable lengths are far shorter than for serial, ranging from 6” to 15”.

Unlike serial communication, parallel communication transmits a full byte of data simultaneously over 8 data lines. Additional hardware lines are used to signal to the receiving side that data has been loaded onto the data lines and ready to be read. Assuming that a parallel port is running with the same clock speed as a serial port, it has the potential of transmitting 8 times faster. LAVA Parallel cards are still used for legacy printing applications as it is still the common standard for printers and scanners. Additionally some legacy applications may require a security dongle be present on the LPT Port. Parallel communication is very reliable and has the advantage of transmitting data at a far greater speed, but is limited by the number of applications it can be used with.

At LAVA we build a number of different Serial and Parallel card slot types such as PCI, PCIe, and ISA that come in a variety of form factors both full height and low profile. These include serial only cards, parallel only cards, and serial/parallel combination cards. Serial and parallel technology both provide a time tested method for transmitting data between applicable devices, and remain an essential communication standard for devices around the world.