RS-422/485 SERIAL COMMUNICATION OVERVIEW

Data is typically transmitted between two points either asynchronously or synchronously. However, what happens to the data between point A and point B is another discussion. This in-between area is a cable made up of wires through which data travels. Specifications for this cable were developed to maximize signal integrity (to limit the possible degradation that could be caused by external noise or ground shifts).

BALANCED TRANSMISSION

The RS-422 protocol greatly expands the practical possibilities of the serial bus. It provides a mechanism by which serial data can be transmitted over great distances (to 4,000 ft) and at very high speeds (to 10 Mbps). This is accomplished by splitting each signal across two separate wires in opposite states -- one inverted; the other not inverted.

The difference in voltage between the two lines is compared by the receiver to determine the logical state of the signal. This wire configuration, called differential data transmission, this potential difference will affect both wires equally, and thus not effect their inverse relationship. Twisted pairs of wire, which ensure that neither line is permanently closer to a noise source than the other, are often used to best equalize influences on the two lines. Errors can be caused by high noise levels affecting one side of the receiver to a different extent than the other. To combat this, each receiver is generally grounded.

Errors in balanced transmission systems such as RS-422 can be also caused by signal reflections. As data transfer speeds increase and travel over longer distances, the signal can be reflected back from the far end of the wire. To combat this, termination resistors are placed at the far end of the cable which make the cable appear electrically as if it is indefinitely long -- indefinitely long lines don't have ends, thus can't reflect from one end to the other. These termination resistors will differ depending on the protocol used. For RS-422 a 100 ohm resistor is placed at the receiving device.

FULL-DUPLEX, HALF-DUPLEX AND MULTIPORT SYSTEMS

An RS-232 based system allows only two devices to communicate. With RS-422 a master can use one communication line to converse with up to 10 slaves. With that many parties wanting to talk, a mechanism for controlling the conversation must be implemented. Two such mechanisms exist:

Full-Duplex and Half-Duplex

In Half-Duplex operation, communication can take place in only one direction at a time. This is an obvious solution to a balanced system employing only one set of wires -- if signals were coming in both directions at once, they would conflict with each other. However, in even systems in which different pairs of wire are used for transmission and reception can still operate in Half-Duplex mode.

In Full-Duplex systems, transmission and reception can occur at the same time. Thus Point A can send information to Point B while at the same time receiving data from Point B. Full-Duplex operation becomes especially important in systems where a single master is communicating with multiple receivers. With a Full-Duplex configuration, Point A can send data to Point B while receiving data from Point C.

RS-485: THE TRUE MULTIDROP NETWORK

RS-485 is an upgraded version of the RS-422 protocol that was specifically designed to address the problem of communication between multiple devices on a single data line. It is a balanced transmission system that is virtually identical to RS-422 with the important addition of the ability to allow up to 32 devices to communicate using the same data line. Thus any Point A through Point FF can directly communicate with each other, taking on the role of master and slave as needed. This is achieved with tristatable drivers which are usually controlled by a programmable handshake line to ensure that only one device acts as a driver at one time.

In such a system, the RS-485 line cannot be thought to have a beginning and an end, because communication can be initiated from any point on the line. Thus, terminating resistors must be placed at both ends of the RS-485 wire to achieve the infinite line illusion. For RS-485, 120 ohm resistors are placed at the two furthest points of the communication link.