RS485 Data Interface

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RS-485 
BALANCED DIFFERENTIAL DRIVERS
Balanced Line Drivers

RS485  
is a specialized interface that would not be considered standard equipment on today's home PC but is very common in the data acquisition world. RS232 is the most common interface used to communicate serially but it has it's limitations.

Standards have been developed to insure compatibility between units provided by different manufacturers, and to allow for reasonable success in transferring data over specified distances and/or data rates. The Electronics Industry Association (EIA) has produced standards for RS485, RS422, RS232, and RS423 that deal with data communications. Suggestions are often made to deal with practical problems that might be encountered in a typical network. EIA standards where previously marked with the prefix "RS" to indicate recommended standard; however, the standards are now generally indicated as "EIA" standards to identify the standards organization. While the standards bring uniformity to data communications, many areas are not specifically covered and remain as "gray areas" for the used to discover (usually during installation) on his own

RS485 will support 32 drivers and 32 receivers (we are talking about bi-directional - half duplex - multi-drop communications over a single or dual twisted pair cable !!).  An RS-485 network can be connected in a 2 or 4 wire mode.  Maximum cable length can be as much as 4000 feet because of the differential voltage transmission system used. The typical use for RS485 is a single PC connected to several addressable devices that share the same cable. You can think of RS485 as a "party-lined" communications system (the addressing is handled by the Remote Computer unit). The RS232 may be converted to RS485 with a simple interface converter - it can have optical isolation and surge suppression.

RS232 signals are represented by voltage levels with respect to system common (power ground). This type of signal works well in point to point communications at low data transmission rates. RS232 ports on the PC are assigned to a single device. COM1 could be the mouse port and COM2 used for a modem. This is a example of point to point (one port communicates with one device). RS232 signals require a common ground between the PC and the associated device. Wiring distances should be limited to one or two hundred feet on async. data and about 50 feet with sync. data (that may be pushing things in some cases). Synchronous data has a transmit and receive clock that limits the max distance you can go on a sync. data line

In short, the RS232 port was designed to communicate with local devices, and will support one driver and one receiver.

SPECIFICATIONS RS485
Mode of Operation DIFFERENTIAL
Total Number of Drivers and Receivers on One Line 1 DRIVER
32 RECEIVER
Maximum Cable Length 4000 FT.
Maximum Data Rate 10Mb/s
Maximum Driver Output Voltage -7V to +12V
Driver Output Signal Level (Loaded Min.) Loaded +/-1.5V
Driver Output Signal Level (Unloaded Max) Unloaded +/-6V
Driver Load Impedance (Ohms) 54
Max. Driver Current in High Z State Power On +/-100uA
Max. Driver Current in High Z State Power Off +/-100uA
Slew Rate (Max.) N/A
Receiver Input Voltage Range -7V to +12V
Receiver Input Sensitivity +/-200mV
Receiver Input Resistance (Ohms) >=12k

SELECTION OF TRANSMISSION LINE
FOR  RS-485

When choosing a transmission line for RS-485, it is necessary to examine the required distance of the cable and the data rate of the system.  Losses in a transmission line are a combination of ac losses (skin effect), dc conductor loss, leakage, and ac losses in the dielectric. In high quality cable, the conductor losses and the dielectric losses are on the same order of magnitude.

CABLE SELECTION FOR RS-422 AND RS-485 SYSTEMS

Selecting data cable for an RS-422 or RS-485 system isn't difficult, but often gets lost in the shuffle of larger system issues. Care should be taken, however, because intermittent problems caused by marginal cable can be very difficult to troubleshoot.

Beyond the obvious traits such as number of conductors and wire gauge, cable specifications include a handful of less intuitive terms.

Characteristic Impedance (Ohms): A value based on the inherent conductance, resistance, capacitance and inductance of a cable that represents the impedance of an infinitely long cable. When the cable is cut to any length and terminated with this Characteristic Impedance, measurements of the cable will be identical to values obtained from the infinite length cable. That is to say that the termination of the cable with this impedance gives the cable the appearance of being infinite length, allowing no reflections of the transmitted signal. If termination is required in a system, the termination impedance value should match the Characteristic Impedance of the cable.

Shunt Capacitance (pFft): The amount of equivalent capacitive load of the cable, typically listed in a per foot basis. One of the factors limiting total cable length is the capacitive load. Systems with long lengths benefit from using low capacitance cable.

Propagation velocity (% of c): The speed at which an electrical signal travels in the cable. The value given typically must be multiplied by the speed of light (c) to obtain units of meters per second. For example, a cable that lists a propagation velocity of 78% gives a velocity of 0.78 X 300 X 10' - 234 X 106 meters per second.


Plenum cable

Plenum rated cable is fire resistant and less toxic when burning than non-plenum rated cable. Check building and fire codes for requirements. Plenum cable is generally more expensive due to the sheathing material used.

The RS-422 specification recommends 24AWG twisted pair cable with a shunt capacitance of 16 pF per foot and 100 ohm characteristic impedance. While the RS-485 specification does not specify cabling, these recommendations should be used for RS-485 systems as well.

It can be difficult to quantify whether shielding is required in a particular system or not, until problems arise. We recommend erring on the safe side and using shielded cable. Shielded cable is only slightly more expensive than unshielded.

There are many cables available meeting the recommendations of RS-422 and RS-485, made specifically for that application. Another choice is the same cable commonly used in the twisted pair Ethernet cabling. This cable, commonly referred to as Category 5 cable, is defined by the EIA/TIA/ANSI 568 specification. The extremely high volume of Category 5 cable used makes it widely available and very inexpensive, often less than half the price of specialty RS422/485 cabling. The cable has a maximum capacitance of 17 pF/ft (14.5 pF typical) and characteristic impedance of 100 ohms.

Category 5 cable is available as shielded twisted pair (STP) as well as unshielded twisted pair (UTP) and generally exceeds the recommendations for RS-422 making it an excellent choice for RS-422 and RS-485 systems.