In most any data transfer from device to controller or machine to machine, the link must be reliable. Even with the exponential growth of Ethernet as the backbone of factory and enterprise data transfer, the serial communications link is still the workhorse in digital data transfer.
The Electronic Industries Assn. (EIA) RS-232 standard is the oldest standard for common serial communication. It was developed in 1962 and quickly became an expected port on data handling equipment well before it became ubiquitous on PCs.
RS-232 is used for inexpensive, relatively slow point-to-point communications over short distances. The signals are represented by voltage levels with respect to a system common (power/logic ground). Data rates of up to 230 kbits/sec. (special chips support faster speeds than the original 19.2 kbits/sec. specification) are achievable, but speed decreases with distance, which taps out at about 75 ft., although distances approaching 150 ft. with acceptable speed may be possible with CAT 5 low-capacitance cable.
Cable length can be extended by means of optical isolators and other signal boosters. Optical isolators with photo diodes isolate each wire including the ground in a serial cable, so noise affects each line equally. The voltage on the signal lines then maintains the true voltage-relative to the ground-negating the noise. The RS-232 port was only designed to communicate with local devices and will support one driver and one receiver.
When communicating at higher data rates or over longer distances, these single-ended methods are not enough. The balanced differential signal in the RS-422 serial communications standard offers superior performance in most applications. Differential signals can help nullify the effects of ground shifts and induced noise signals that can appear as common mode voltages on a network.
Where the RS-232 has a signal and ground, the RS-422 standard uses two lines to transmit data. The standard extends serial communications up to 4,000 ft. at 115 kbits/sec. between two devices. Each channel of communication is converted to a differential signal and carried on a twisted-wire pair, so when the differential signal is received, noise is abated.
RS-422 is also specified for multidrop, sometimes called party-line, applications, where only one driver transmits on a trunkline with up to 10 receivers. While a multidrop application such as this has many advantages, RS-422 devices can't be used to construct a true multipoint network. A true multipoint network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. The connector pin assignments for RS-422 differ from RS-232, so even thought the connector looks the same, the two standards are incompatible.
The RS-485 standard is a specialized interface that is not typically found on today's home PC, but is very common in the data-acquisition world. RS-485 is an extension of the RS-422 standard, allowing true multipoint communications between devices on a single wire. Speed and distance capabilities are the same as RS-422, but up to 32 drivers and receivers can be handled on a single two-wire bus, and this can be extended with serial repeaters or special drivers to thousands of nodes. RS-485 chipsets have the ability to turn their transmitters on and off quickly, so multiple transmitters can be attached to the network and activated as necessary.
Protocol converters enable connectivity between devices that operate on different communications protocols. RS-232 devices can be connected to RS-485 devices using RS-485 protocol converters. They are used primarily to connect to specialized equipment that still uses an older protocol.
Converters also extend the distance limitation of a standard PC serial port (which is typically RS-232 and limited to 16 meters) to the 1,200 m capability of RS-485.
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Serial Ports Made Simple
Industry Designator....................RS-232.....RS-422.....RS-485
Nominal Speed (kbits/sec.)...........115...........115...........115
Nominal Distance..........................75..........4,000........4,000
