How Serial Ports Work

The serial port has been an integral part of most computers for more than 20 years. Serial ports provide a standard connector and protocol to let you attach devices, such as modems, to your computer.All computer operating systems in use today support serial ports. Parallel ports are a more recent invention and are much faster than serial ports.


The serial port has been an integral part of most computers for more than 20 years. Serial ports provide a standard connector and protocol to let you attach devices, such as modems, to your computer.

All computer operating systems in use today support serial ports. Parallel ports are a more recent invention and are much faster than serial ports. USB ports are only a few years old, and will likely replace both serial and parallel ports over the next several years.

The serial port transmits the eight bits in a data byte one bit at a time. The advantage is that a serial port requires only one wire to transmit the eight bits, while a parallel port requires eight. The disadvantage is that it takes eight times longer to transmit the data than it would if there were eight wires. Before each byte of data, a serial port sends a start bit. After each byte of data, it sends a stop bit to signal that the byte is complete. It may also send a parity bit.

Serial ports are bidirectional. Bidirectional communication allows each device to receive data as well as transmit it. Serial devices use different pins to receive and transmit data allowing full-duplex communication. Using the same pins would limit communication to half-duplex, meaning that information could only travel in one direction at a time.

Serial ports rely on the universal asynchronous receiver/transmitter (UART) to function properly. The UART chip transforms the parallel output of the computer's system bus into serial form for transmission through the serial port.

The serial connection

The external connector for a serial port can be either 9 pins or 25 pins. Originally, the primary use of a serial port was to connect a modem to the computer. The following list describes the function of each pin of a 9-pin serial port when a modem is connected.

  • Pin 1. Carrier Detect — Determines if the modem is connected to a working phone line.

  • Pin 2. Receive Data — Computer receives information sent from the modem.

  • Pin 3. Transmit Data — Computer sends information to the modem.

  • Pin 4. Data Terminal Ready (DTR) — Computer tells the modem that it is ready to talk.

  • Pin 5. Signal Ground — Pin is grounded.

  • Pin 6. Data Set Ready (DSR) — Modem tells the computer that it is ready to talk.

  • Pin 7. Request To Send (RTS) — Computer asks the modem if it can send information.

  • Pin 8. Clear to Send (CTS) — Modem tells the computer that it can send information.

  • Pin 9. Ring Indicator — Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected.

    • Voltage applied to the pins can be either on or off. "On" (binary value 1) means that the pin is transmitting a signal between -3 and -25 V, while "off" (binary value 0) means that it is transmitting a signal between +3 and +25 V.

      Going with the flow

      Flow control is the ability of one device to tell another device to stop sending data for a while. The commands request to send (RTS), clear to send (CTS), data terminal ready (DTR), and data set ready (DSR) are used to enable flow control.

      A modem that communicates at 56 kbps is an example of how flow control works. The serial connection between the computer and modem transmits at 115 kbps. This means that the modem is getting more data coming from the computer than it can transmit over the phone line. Even if the modem has a 128K buffer to store data in, it will still quickly run out of buffer space and be unable to function properly with all that data streaming in.

      With flow control, the modem can stop the flow of data from the computer before it overruns the modem's buffer. The computer is constantly sending a signal on the RTS pin, and checking for a signal on the CTS pin. If there is no CTS response, the computer stops sending data, waiting for the CTS before it resumes. This allows the modem to keep the flow of data running smoothly.

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Pipe fabrication and IIoT; 2017 Product of the Year finalists
The future of electrical safety; Four keys to RPM success; Picking the right weld fume option
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Power system design for high-performance buildings; mitigating arc flash hazards
VFDs improving motion control applications; Powering automation and IIoT wirelessly; Connecting the dots
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me