Know your serials

By Loren Shaum

By definition, serial communication interface is a connection and wiring configuration that allows the exchange of data between various intelligent devices, one bit at a time. Serial communication interfacing (SCI) was created in the ‘60s, but coined as such by Motorola in the ‘70s. Today, it is a relatively standard method of communicating on the factory floor--particularly in legacy machine control systems.

The basic SCI contains a parallel-to-serial converter, which serves as the transmitter, and a serial-to-parallel converter that serves as a receiver. Because these two electrical circuit configurations almost always are located on separate devices or equipment, they are clocked independently. Interrupt and enable signals are the means of alerting each other that data is coming. Data speed is programmable, so receivers must be configured to match the transmitter’s transmission rate. Other characteristics include a non-return-to-zero format and the ability to operate in half-duplex mode. The latter uses only the receiver or only the transmitter; full-duplex mode uses both transmitter and receiver simultaneously.

The primary advantages of serial communication interfaces for machine builders are the simple wiring and the cost as compared, for example, to parallel communication interfaces. Also, serial communication interface cables can be much longer than parallel cables because of less cross-talk.

Many Serial Standards
The specific connectivity of the various serial communication interfaces varies depending which Electronics Industry Association (EIA) standard is incorporated. Of these, the most common interfaces used by machine builders are 232, 422 and 485. In 1997, EIA designated each as EIA232, EIA422 and EIA485, respectively. In spite of the new designation, most all documents today still use the ‘RS’ prefix.

EIA 232

	9-pin D-sub Connector

The EIA232 standard is the most common serial interface used in machine automation. The most recent version is EIA232F. This change renamed some of signal lines and added a shield connection. The table below represents current EIA232 specifications. Today, connections to and from various devices vary dramatically. Perhaps the most common connection is through the 9-pin D-sub connector shown here. Another connection commonly used on machine control equipment is the 25-pin D-sub connector.

EIA422
EIA422 is a balanced, differential serial interface using twisted wire pairs. The advantage of a balanced signal is greater noise immunity. EIA describes EIA422 as a transmitter to receiver interface for point-to-point connections.  EIA422 was created for greater distances and higher Baud rates than EIA232. EIA422 is also specified for multi-drop applications where only one driver is connected to, and transmits to, as many as ten receivers. The interface is normally terminated on a four-wire screw block or on a Sub-D37 connector, but other interfaces are not uncommon.

EIA422 Specifications