Serial communication is the process of transferring data one bit at a time, as opposed to parallel communication, where several bits are sent at the same time over multiple parallel lines. One could consider serial communication analogous to Morse code.
There are many types of serial communication (RS-422, RS-485, Ethernet, USB, etc.), but for the purpose of this white paper we will be focusing on RS-232 – a relatively simple, inexpensive and standardized means of data transfer and control that requires minimal supporting software from the host device.
On a very basic level, a serial cable only requires three signals: Transmit, Receive, and Signal Ground (see below for common abbreviations). Many devices also require additional control signals to regulate the flow of informationin in order to prevent potential data loss. All valid signals are defined as positive or negative voltage relative to the common Signal Ground.
The original Electronic Industries Association (EIA) standard RS-232-C recommended but did not require the use of DB-25 connectors in part because of the 20 different signal connections. As very few devices require more than a handful of these connections, many computer manufacturers have opted for smaller connectors. The most popular of these is the DE-9, commonly (and incorrectly) referred to as DB9. Either or both of these connectors can be used for a serial cable.
There are two general types of RS-232 serial cables: standard straight through cables (wired 1-1, 2-2, 3-3, etc.), and null modem cables. Null modem cables are not mentioned in the standard, so there is no official pinout for them. Most RS-232 applications only require seven to nine wires, though a DB25 cable with all pins connected can also be used for RS-530 or parallel printer connections if twisted pair cable is used and it is wired correctly. Below is a list of RS-232 signals and their pin assignments.
Transmit Data (a.k.a. TD, Tx)
Receive Data (a.k.a. RD, Rx)
Request To Send
Clear To Send
Data Set Ready
Signal Ground (a.k.a. SG)
Data Carrier Detect (a.k.a. CD)
Secondary Carrier Detect
Secondary Clear To Send
Secondary Transmit Data
Transmit Clock (a.k.a. TCLK, TxCLK)
Secondary Receive Data
Receive Clock (a.k.a. RCLK, RxCLK)
Secondary Request To Send
Data Terminal Ready
Remote Loopback, Signal Quality Detector
Signal Rate Selector
Auxiliary Clock (a.k.a. ACLK)
The maximum length of a serial cable depends on a number of factors. These factors include various characteristics of the transmitters and receivers, baud rate, and the capacitance and impedance of the cable. The RS-232 standard requires that a serial port provide signals appropriate for a capacitive load of 2,500 pF (picofarads). Therefore, using a low capacitance cable will allow signal integrity to be maintained at a greater distance. Properly shielded and grounded cable is also recommended, though for some installations the potential for causing ground loops should be considered.