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RS-232 Explained

Serial Communication Standards and the Need for Serial to USB Adapters

rs-232 serial to usb adapter

The RS-232 Standard

RS-232 (originally called Recommended Standard 232 but shortened to RS232) was developed by the Electronic Industry Association (EIA) in the 1960s for the exchange of data between DTE (Data Terminal Equipment) and DCE (Data Communication Equipment). It was later adopted by the personal computer industry so PCs could connect to modems, peripherals and data acquisition devices such as lab test equipment, GPS receivers and point of sale terminals.

RS-232 is a simple serial communication standard in which one bit of data is transmitted at a time over a serial cable at the relatively slow rate of up to 20K bits/second and distances up to 50-ft. By specifying the voltage, signaling, pin wiring and control data between a host device and a peripheral, the RS-232 standard ensured that devices from different manufacturers could communicate with each other.

A serial port was standard on personal computers until the late 1990s, when it was mostly replaced by the Universal Serial Bus (USB). It is still common on legacy devices that use serial communications for data capture, configuration and control. Rather than replace these devices (if replacement is even possible), an inexpensive way to extend their life is to use a serial to USB adapter to connect to a modern laptop's USB-A or USB-C port.

Looking for an RS232 to USB adapter that actually works?

Eaton's Keyspan RS232 to USB converter (Model Number USA-19HS) is widely recommended by serial device manufacturers for its compatibility and reliability.

Keyspan USB to Serial Adapter

connect 9-pin (DB9) serial device to computer or laptop USB-A port

The Keyspan USB to Serial Adapter is also available for USB-C (Model Number USA-19HS-C).

What is the difference between RS232 and RS422?
Introduced in 1975, the RS-422 standard was intended as a replacement for RS-232C. It used lower voltages and differential signaling to support faster data transmission, longer cable lengths and reduced RF/EMI noise. It specified the same DB25 connector as the original RS-232 standard.

RS-422 was used with a smaller 8-pin mini-DIN connector on early Apple Macintosh computers to connect to AppleTalk networks and various peripherals. RS-422 is also found on professional video broadcasting equipment.

What is the difference between RS232 and RS485?
The RS-485 standard was published in 1983 and is substantially similar to the earlier RS-422 standard. The big difference is support for multiple transmitters and receivers, allowing devices connected on an RS-485 network to "talk amongst themselves" in half-duplex mode over inexpensive twisted pair cabling. RS-485 describes the electrical characteristics of the drivers and receivers but does not specify a communication protocol.

RS-485 has been widely used as the physical layer for standard and proprietary protocols in industrial control systems, including Modbus and Allen Bradley DH 485.

RS-232 RS-422 RS-485
Bus Topology Point-to-Point Multidrop Multidrop
Transmission Mode Full Duplex Full Duplex Half Duplex (2-wire)
Full Duplex (4-wire)
Signaling Unbalanced Balanced / Differential Balanced / Differential
Number of Transmitters (TX) 1 1 32
Number of Receivers (RX) 1 10 32
Table 1: RS232, RS422 and RS485 Standards Compared

Point-to-Point – a connection between two devices in which bits are transmitted sequentially, one at a time.

Multidrop – a type of bus in which multiple devices are connected at the same time. Through a process of arbitration, one device sends and other devices listen for data packets intended for them.

Full-Duplex – simultaneous data transmission in both directions.

Half-Duplex – data transmission in both directions but not simultaneously.

Unbalanced vs. Balanced – A balanced transmission requires three wires, two signal wires and a ground. Both signal wires carry the same signal, with one signal the inverse of the other (this is referred to as Differential Signaling). In an unbalanced transmission, the signal is carried on one conductor and a second conductor is used for ground.

RS232 Connectors

The D-Subminiature or D-Sub connector is named for the D-shaped metal shield that protects the pins and ensures the correct orientation. First introduced by Cannon in 1952, the D-sub form factor was adopted for the RS-232 serial communications standard.

In the Cannon part numbering scheme, "D" indicated the shape of the shield surrounding the pins, followed by a letter indicating the size of the connector shell (A = 15 pins, B = 25 pins, C = 37 pins, D = 50 pins and E = 9 pins). For example, a DE-9 is a D-sub with a 9-pin or socket shell (although the DE-9 is often mistakenly referred to as a DB-9 connector).

25-Pin Type D Subminiature (DB25)

25-pin type d subminiature / DB25
DB25 connector

The 25-pin D-Sub was the only connector specified in the original RS-232 standard. The 25 pins handled the data transmission, control signaling and grounding required by the relatively simple applications and devices of the time.

9-Pin DE-9 (DB9)

9-pin DE-9 / DB9
DB9 connector

Many serial devices did not implement all 20 control signals in the original RS232 specification. Smaller, simplified connectors with fewer pins, like the DB9, quickly emerged.

DE-15 Connector (DB15)

DE-15 / HD-15 / DB15
DB15 connector

The DE-15 connector (also referred to as an HD-15 or DB15) is used on older model personal computers, projectors and monitors for analog video (VGA, SVGA and XGA). It was also used as a serial connector for game controllers (e.g. joysticks).

RS-232 Signaling

The RS232 standard defines voltage levels for logical 1 and logical 0. Valid signals are in the range of +3V to +15V (logical zero) or −3V to −15V (logical one) as compared to the 0V ground (GND) pin. On most PCs, a signal level of ±13V is typical for RS232 and ±5V for RS485. Signals between -3V and +3V are invalid.

Devices, such as computers and printers, use control signals to synchronize the transmitting and receiving of data. These signals indicate the status of Sender and Receiver (ready, busy, transmitting and receiving) and control the flow of data. For example, the signals supported by a DB9 connector are:

1 Data Carrier Detect (DCD) 6 Data Set Ready (DSR)
2 Receive Data (RxD) 7 Request to Send (RTS)
3 Transmit Data (TxD) 8 Clear to Send (CTS)
4 Data Terminal Ready (DTR) 9 Ring Indicator (RI)
5 Signal Ground (GND)    

Data Carrier Detect (DCD): The Receiver (e.g. modem) tells the Sender (e.g. computer) it is connected to a working telephone line.

Receive Data (RxD): Receiver receives data from the Sender.

Transmit Data (TxD): Data from the Sender to the Receiver.

Data Terminal Ready (DTR): The Receiver tells the Sender it is ready to send or receive data.

Signal Ground (GND): This is the common ground connection for all signals.

Data Set Ready (DSR): The Receiver tells the Sender it is ready to receive.

Request to Send (RTS): The Receiver asks the Sender if it is ready to transmit.

Clear to Send (CTS): The Sender tells the Receiver it is ready to send.

Ring Indicator (RI): Used by older modems but no longer required.

A standard serial port transmits at 115 Kbps. This may be faster than the connected device can handle. Using flow control (RTS and CTS), the device can temporarily stop the flow of data from the computer so it can process the data in its buffer.

RS232 Cable Length and Data Rate

RS232 cables can be simple parallel wires or twisted pair up to 15 m (50 ft.), and 1,200 m (4,000 ft.) for RS422 and RS485 applications.

  RS-232 RS-422 RS-485
Max. Cable Length 15 m (50 ft.) 1200 m (4,000 ft.) 1200 m (4,000 ft.)
Max. Data Rate* 20 Kbps 10 Mbps 50 Mbps
* Data rate depends on the length of the cable. Speeds shown are at maximum cable length. Longer distances can be achieved at lower speeds.

For all serial communication, there is an inverse relationship between cable length and data transmission speed. At the maximum length of 15 meters (50 feet), an RS-232 port will support a data rates up to 20 Kbps. Greater distances can be achieved with a slower data rate and shielded CATx cable (see RJ45 serial adapters).

Serial to USB Adapters

Serial COM ports have been largely replaced by smaller, faster USB-A and USB-C ports on desktop PCs, laptops and peripherals. Computer peripherals such as printers have also adopted the USB interface or gone wireless. However, some network switches, routers and serial devices still use a serial interface, including lab equipment, barcode readers, industrial controllers and data gathering sensors. To connect a legacy serial device with a RS232, RS485 or RS422 port to a modern computer, you will need a serial to USB adapter.

How a Serial to USB Adapter Works
Serial to USB adapters are typically compliant with the USB 2.0 specification, with a data rate of 12 Mbps and support USB features such as suspend (device sleeps), resume (device wakes) and remote wake-up (device wakes host). When a host computer boots up, it assigns a port address to each device (including any USB adapters) connected to the bus and determines the type of data transfer the device wants to perform. The same process is followed when a serial to USB adapter is "hot plugged" into to a USB-A or USB-C port. Once the adapter is registered, it looks like a USB peripheral device to the host computer. On the serial side, the adapter looks like a standard serial COM port with serial data input (SIN) and serial data output (SOUT).

When you plug a USB to Serial Adapter like the USA-19HS into a Windows 10 machine, it should appear in the Device Manager under Ports (COM & LPT). If you do not see this heading, go to the View menu and make sure Show Hidden Devices is selected. If you still do not see your adapter, you will need to install the manufacturer's driver for your device.

USB to serial adapters use a chipset to bridge between the USB port and a UART serial port. The chipset creates an interface between a legacy serial device and a computer with a USB port, allowing the computer to read and write to the adapter using USB commands and the serial device to send and receive serial signals. Since RS232 devices signal using higher voltages, the adapter also needs a "level shifter" to boost the signal voltage up to the level expected by the RS232 receiver and a transient suppressor to protect the USB circuitry from damage.

Serial Communication Q&A

What is the difference between a serial cable and a parallel cable?
Serial cables transmit one bit at a time across a single wire. Each bit in a packet of data is sent sequentially with a start and stop bit at the beginning and end of each packet. Parallel cables transmit all bits in a packet at the same time, one per wire. For an 8-bit byte, the parallel cable would need 8 wires. The receiving device reassembles the packet so the bits appear in the correct order. Parallel cables are much faster than serial cables but they are bulkier, more expensive and prone to signaling errors like crosstalk, especially over longer distances.

What is a COM port?
COM port is another name for a serial port on a computer. Computers no longer have COM port but the driver for a serial to USB adapter may create a virtual COM port so a legacy serial device can connect to the computer.

What is a UART serial port?
Serial ports rely on a microcontroller chip called a Universal Asynchronous Receiver Transmitter (UART) to "serialize" the data coming from the host computer's bus so it can be transmitted through the serial port. Serializing means transforming bytes of data into a sequence of bits with start and stop bits on either end. On the receiving end, another UART chip reconverts the serialized data into "parallel" bytes. Most UART chips cache data coming in from the computer bus so they can transmit faster than the normal 115 Kbps transmission rate of a standard serial port.

What does "baud rate" mean?
Baud rate is the data transmission speed measured in bits per second. It defines the frequency of each bit period. For a baud rate of 2400 (2400 bps), the frequency is 2400Hz and the bit period is 1/2400. This is the information that a receiver uses to recover the bits from the data stream.

What does the "C" mean in RS-232C?
Since the original RS-232 standard was published in 1962, it has been revised a number of times. Each revision is given a letter suffix. The current version, TIA/EIA-232-F, was published in 1997.

What is a null modem cable?
Standard RS232 cables connect a DTE (Data Terminal Equipment) such as a desktop computer to a DCE (Data Communications Equipment), typically a modem, printer or special-purpose peripheral. These "straight through" serial cables connect pins on the sending device to the same pins on the receiving device (e.g. pin 1 to pin 1, pin 2 to pin 2 and so on). Serial cables can also be used to connect two DTEs together but the pins must be wired so transmitter pins are connected to receiver pins. Cables that use this "crossover" wiring are known as null modem cables.

What is an FTDI Adapter?
A USB to serial adapter that uses the FTDI chipset. The chipset manufacturer, Future Technology Devices International (FTDI), is a Scottish semiconductor company specializing in USB interfaces.

Does the Eaton USA-19HS maintain a persistent Windows COM port number?
To function, some devices require a specific COM port number or a COM port in a certain range. You can specify which COM port to use in the Windows Device Manager. Eaton's USA-19HS maintains the same Windows COM port number each time it is plugged in, even if moved to a different USB port.

Products Mentioned in this Article

p520006 6 ft serial DB9 serial extension cable

Serial Cables

b090a9m modular serial adapter (DB9 M to RJ45 F)

Serial to RJ45

usa19hs keyspan USB to serial adapter

Serial to USB

p456006 6 ft null modem serial DB9 serial cable

Null Modem Cables

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