This article serves as a general overview of the differences between the following cables and their serial communication interfaces:
The RS (or recommended standards) are set for general usage on serial (wired) communications systems. It does not include or exclude any specific types of communication or communication protocols. In short, you can use these different wires and/or interfaces to speak to a multitude of different products with unique communication protocols and/or languages.
Note: BACnet is a communications protocol for Building, Automation and Control (BAC) networks that defines the specs for MS/TP (Master Slave/Token Passing) relationships between devices. It is considered an ISO global standard and is accepted by several US national standards organizations, as well as other regulatory organizations in more than 30 countries.
Here's what defines a BACnet-approved cable:
“An MS/TP EIA-485 network shall use shielded, twisted-pair cable with characteristic impedance between 100 and 130 ohms. Distributed capacitance between conductors shall be less than 100 pF per meter (30 pF per foot). Distributed capacitance between conductors and shield shall be less than 200 pF per meter (60 pF per foot). Foil or braided shields are acceptable. The maximum recommended length of an MS/TP segment is 1200 meters (4000 feet) with AWG 18 (0.82 mm2 conductor area) cable. The use of greater distances and/or different wire gauges shall comply with the electrical specifications of EIA-485.”
The RS-485 (aka, TIA-485(-A), EIA-485) is a standard for defining the electrical characteristics of drivers and receivers when used for serial communications systems. RS-485 is designed to be a balanced system. Meaning that there are 2 wires (other than ground) used to transmit signals.
It is generally accepted that RS-485 can be used with data rates up to 10 Mbps for distances up to 1,200 m (4,000 ft). As a rule of thumb, the speed in Mbps multiplied by the length in meters should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbps.
RS-422 (aka, TIA/EIA-422) is a technical standard that can transmit data at rates as high as 10 Mbps for as long as 1,200 meters (3,900 ft) at lower rates.
The RS-422 specification recommends 24AWG twisted pair cable with a shunt capacitance of 16 pF per foot and 100 ohm characteristic impedance.
The RS-232 is a standard cable that was commonly used in the older DB-25 computer connectors:
The RS-232 cables can be used on distances up to about 300 m (1,000 ft). Other signal standards (such as RS-422) are better suited for longer distances.
Cat-5 / Cat-6
Ethernet cabling (commonly referred to as Category 5 or 6 cables) is defined by the EIA/TIA/ANSI 568 specification. The extremely high volume of Category 5 cable availability makes it very inexpensive for the installer (often less than half the price of specialty RS-422/485 cabling). It has a maximum capacitance of 17 pF/ft (14.5 pF typical) and a characteristic impedance of 100 ohms.
Category 5 cable is available in STP (shielded twisted pair) as well as UTP (unshielded twisted pair) and generally exceeds the recommendations for RS-422 making it an excellent alternate for RS-485 communications.
What Is Twisted Pair, and Why Is It Used?
As its name implies, a twisted pair is simply a pair of wires that are of equal length and are twisted together. The wiring of the RS-485 connection requires a cable that includes at least three wires.
For connection with CCA, Tigo recommends using an RS-485 communication data cable with two or more twisted pairs.
It can be difficult to quantify whether shielding is required in a particular system or not until communication problems arise. Since shielded cable is only slightly more expensive than unshielded, Tigo recommends erring on the safe side. There are many cables available that meet the RS-422 and RS-485 specs.
What is a Plenum-Rated Cable?
Plenum-rated cable is a cable that is specifically designed to be installed within a plenum (an empty space within a building that facilitates HVAC, air ducts, pipes, and electric wiring). These cables are manufactured to be fire-resistant and less toxic when burning. They can be generally more expensive than non-plenum-rated cables due to the sheathing material used.