Types of serial cable configuration

What is the serial cable?

A serial cable is a data transfer cable that sends data between two devices using the serial communication protocol. The serial port determines the type of connectors to be utilized. The term “null modem cable” refers to a connection that connects two DTEs directly.

Serial ports were standard on early computers for attaching a modem, mouse, and other peripherals, and they are widely utilized by sensors for data gathering. Printers on older Macs were connected via the serial port.

RS232, RS422, and RS485 are the major serial cable protocols used in industrial networking.

RS 232

The RS-232 interface adheres to the Electronic Industries Alliance’s interface standard for serial data transfer (EIA). EIA-RS-232 is the initial number (232, RS232 for short). It’s commonly used to connect computer serial interface peripherals. Connect cables and processes such as mechanical, electrical, signal, and transfer.


The RS-232-C standard specifies data transfer rates of 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, and 19200 baud per second.

One of the most widely used communication standards today is RS-232. RS-232 ports may be found on most personal computers until recently. Instead, you’re more likely to see USB ports now. Naturally, there are plethoras of RS-232 to USB converters on the market.

Cable pinout of RS 232

Use of RS 232

  • Used to connect PLCs to HMIs, I/Os, motor drives, etc
  • Used to transfer serial binary data between devices

Cable color code of RS232

Advantages and Disadvantages of RS 232

Advantages of RS 232

  • Mainly used to connect point-to-point connection between DCE & DTE device
  • Low cost
  • Noise-free
  • RS 232 supports various devices due to its simple structure

Disadvantages of RS 232

  • Low Bandwidth
  • Data can be transferred at only 20 Kbps
  • High power consumption
  • Limited cable length (about 50 feet)

RS 485

When communication distances of many tens of meters to several kilometers are required, the RS-485 serial bus is extensively utilized. To reject common-mode interference, RS-485 uses balanced transmit and differential receptions. The bus transceiver has great sensitivity and can detect voltages as low as 200mV, allowing it to recover the broadcast signal over a kilometer.


RS-485 operates in half-duplex mode, sending only one point at a time. As a result, an enabled signal must regulate the transmitting circuit.

Cable pinout of RS 485

Use of RS 485

  • Used in a wide range of computer and automation
  • Used to transfer the data between the disk drive and the controller
  • In Aircraft also use RS 485 for low-speed transmission of data

Cable color code of RS485

Advantages and Disadvantages of RS 485

Advantages of RS 485

  • Low signal interface
  • Transmission speed of data is maximum
  • High noise immunity
  • Low signal level interface
  • Can be connected up to 1200 meters
  • It supports multiple slaves with a single master
  • Faster communication

Disadvantages of RS 485

  • Only one node can transmit data at a time
  • Need special cable for transferring data
  • Not suitable for large files
  • Backing up of the system is difficult

RS 422

The predecessor of RS-485 is RS-422, and the two are frequently confused.

Because they both use differential signaling, communicate over great distances, and provide multidrop, RS-422 and RS-485 are similar. RS-422 multidrop, on the other hand, allows for only one driver and a maximum of ten receivers.


RS-422 is commonly a four-wire, full-duplex protocol. As a result, RS-422 ports are frequently utilized to connect with full-duplex RS-485 devices.

In the industry, RS-485 has mostly superseded RS-422 as the serial multidrop standard.

Cable pinout of RS422

Use of RS422

With unidirectional/non-reversible, terminated, or non-terminated transmission lines, point to point, or multi-drop, RS-422 allows for data transmission via balanced or differential signaling.

Advantages and Disadvantages of RS 422

Advantages of RS 422

  • High data rates
  • High noise immunity
  • Faster communication
  • Can be connected up to 1200 meters

Disadvantages of RS 422

  • Unidirectional
  • Not widely used

Comparisons between RS232, RS422, RS485

Parameters RS232 RS485 RS422
Network Topology Point-to-point Point-to-point or Multidrop Point-to-point or Multidrop
Number of Device 1 controller & Receiver 32-unit loads (Controllers or Receivers) 1 controller and 10 receivers
Signalling Unbalanced Differential Signalling (Balanced) Differential Signalling (Balanced)
Type of Duplex Full Duplex Half Duplex / Full Duplex Half Duplex Full Duplex
Mark (Data 1) -15 V to -3 V 1.5 V to 5 V (B>A) 2 V to 6 V (B>A)
Space (Data 0) 3 V to 15 V 1.5 V to 5 V (A>B) 2 V to 6 V (A>B)
Max Distance 50 ft @ 19.2 Kbps 4000 ft @ 100 Kbps 4000 ft @ 100 Kbps
Contacts in use TxD, RxD, RTS, CTS, DTR, DSR, DCD, GND DataA, DataB, GND TxA, TxB, RxA, RxB, GND

Types of connectors and cables

  • Coaxial Cable Connectors.

  • Twisted Pair Cable Connectors.

  • Fiber-Optic Cable Connectors.

Coaxial Cable Connectors.

Coaxial cable is a high-frequency signal transmission cable that is also known as coax cable. To eliminate crosstalk from motors, lighting, and other EMI sources, it incorporates a spherical copper conductor with three layers of insulation and shielding. The shield construction of a coaxial cable allows for longer cable lengths between two devices.

Twisted Pair Cable Connectors.

Twisted wires are made up of two twisted wires. Each pair would consist of a wire for the positive data signal and a wire for the negative data signal. Any noise that arises on one of the pair’s positive/negative wires will also appear on the other wire. These are 180 degrees out of phase because the wires have opposite polarity (180 degrees or the definition of opposite polarities). The noise cancels or nulls itself out at the receiving end when it appears on both cables.

Twisted pair cable comes in two varieties:

Shielded Twisted Pair Cable

Twisted pair cables function best in systems that use the balanced line transmission technique. Protected twisted pair cables, abbreviated STP, are twisted pair wires that have been shielded.

Unshielded Twisted Pair Cable

Unshielded twisted pair wires (UTP) are those without shields. The cable’s unique impedance is achieved by twisting the wires together. UTP cable is used for Ethernet.

Fiber-Optic Cable Connectors.

Fiber optic cable, also known as optical fiber cable, is a form of Ethernet cable that contains one or more data-transmitting optic fibers. Fiber optic cable delivers data by sending light pulses through microscopic glass tubes. Optical fiber cable has a transmission capacity 26,000 times more than twisted pair cable.

How to troubleshoot serial cable issues

The common issues in serial cables are

  • Wrong serial cable
  • Software issues
  • Wrong communication parameters
  • Faulty cable
  • Faulty wiring

Wrong serial cable

There are two types of serial port connectors are available in the marker namely 9-pin and 25-pin.

A serial port is a physical connector on the back of a computer that enables data input and output. As a result, a suitable cable and/or adaptor are required at all times to ensure a good connection.

Another feature of a cable that is sometimes overlooked is whether the application requires a “null-modem” or “straight-through” cable. Straight-through cables feature a female connector on one end and a male connector on the other, while null modem cables normally have a female connector on both ends. Make sure you’re using the right cable for your application.

Software issues

A broken serial port can be caused by a software driver. This driver may stop working if it isn’t installed correctly or isn’t compatible with a serial port.

A user can resolve this issue by reinstalling the affected drivers. A user may also be able to change the serial port settings to guarantee that the problem is totally rectified.

Wrong communication parameters

Incorrect communication parameter settings are the most prevalent cause of serial port communication issues. Both devices must be set up with the same communication parameters, such as baud rate, parity, number of data bits, and number of stop bits, in order to work properly.

Faulty cable

Incorrect wiring can be a concern when connecting a control system to a device.

The transmit and ground pins of the control system must typically be linked to the Receive and Ground pins of the connected device, respectively. A third wire may also be attached if a connected system wants to receive a response from a controlled device.

Faulty wiring

The problem can stifle data transfer and retrieval, but it’s simply rectified by swapping out the serial cable.

A loose cable may only need to be reconnected to a serial port in many circumstances. In other cases, however, an inefficient cable must be replaced right away, especially if the serial port is working well.