RS-485 Bus

TIA/EIA-485 Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems

[RS485 Description] [RS485 ICs ] [RS485 Standard Organizations]
[RS449 Cabling] [Pull-Up/Pull-Down Resistors] [RS485 Converter Devices]

EIA-422/485 Bus Description

 EIA-485 Bus Circuit
EIA/TIA-485 Interface Circuit {RS485}

EIA-485 [TIA-485] Balanced (differential) interface; defines the Physical layer, signaling protocol is not defined. EIA485 is also called the RS485 standard, but the term RS485 is out-dated. EIA-485 specifies bidirectional, half-duplex data transmission. Up to 32 transmitters and 32 receivers may be interconnected in any combination, including one driver and multiple receivers (multi-drop), or one receiver and multiple drivers. The maximum of 32 devices is defined based on the Unit Load [UL] of each device [12K ohms]. The maximum devices on the net may be increased if the devices represent less then the UL [fractional unit load]. A number of devices are being produced which represent 1/4 or 1/8 the unit load. A maximum of 256 device could exist on the bus when each is at 1/8 the UL [96k ohms]. Repeaters may be used to extend the net to any number of devices. ProfiBus defines an RS-485 type bus using repeaters.
EIA-485 requires a 120 Ohm line impedance (normally Shielded Twisted pair 'STP'), 120W (10%, 1/2 watt) terminations at both ends of the line (at the Receivers).
Pull-up/Pull-down resistors (Idle-line failsafe) at one end of the 485 bus may be used to bring the line voltage to a steady state (200mV) value at the end of a transmission; when all drivers are in the passive state.
No maximum bus length is given for RS485, but ends up around 1200 meters at 200kbps or 50 meters at 10Mbps. The speed of the system and distance between devices is determined in large amount by the inter-connecting cable. Single board solutions may be hooked up as shown, but chassis to chassis systems require a common ground connection to run between the driver and receiver.
A number of specifications use EIA-485 as the electrical standard, including SCSI-2, SCSI-3, BitBus, and Profibus.

EIA/TIA-422 define a Balanced (differential) interface; specifying a single, unidirectional driver with multiple receivers (up to 32). RS-422 will support Point-to-Point, Multi-Drop circuits, but not Multi-Point [EIA485]. EIA-485 devices may be used in 422 circuits, but EIA-422 may not be used in 485 circuits (because of the lack of an Enable line). EIA422 is the differential "brother" to EIA423. One application note indicated that the combination of cable length (in meters) and data signaling rate (in bps) for RS422 should not exceed 108.
The example provided a maximum speed of 200kbps with a maximum cable length of 500 meter [500 x 200,000]. An EIA-422 driver can support up to 10 loads, at 4K W per load

This page provides two examples of RS485 fault protection.
How close can I place the Bus Nodes.

{RS485 Bus Index}

EIA-485 Standard Organizations

RS485 Specification / RS485 Standard:
TIA/EIA-485: Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems
TIA/EIA-422: Electrical Characteristics of Balanced Voltage Digital Interface Circuits

RS-485 Deterministic TTP Databus Standard; SAE AS6003/2 [Time Triggered Protocol]

Telecommunications Industry Association; TIA []
Note the Electronic Industries Alliance {EIA} ceased operations on Feb 22 2011.

International Specification:
V.11: Electrical characteristics for balanced double-current interchange circuits operating at data signalling rates up to 10 Mbit/s

International Telecommunication Union, ITU []

US Federal Specification:
FED STD 1030A is identical to TIA/EIA-422-B. It is intended for United States, non-military government use.
FED STD 1020A is identical to TIA/EIA-423-B. It is intended for the United States, non-military government use.

{RS485 Bus Index}

EIA-458 Bus Interface IC Manufacturers

RS485 Voltage Switching Levels
RS485 Interface Levels

RS485 IC Vendors:
Dallas Semiconductor Corp, see Maxim for RS485 Line Driver and Receivers

Linear Technology {EIA422-EIA485 Transceiver-Driver-Receiver ICs}

Maxim {RS232, RS485, RS422 IC Manufacturer}

National Semiconductor {RS485 Differential Bus Transceiver-Receiver-Driver}

ON Semiconductor {EIA485 Transceiver, EIA562, EIA422/EIA423 IC Manufacturer}

STMicroelectronics {RS-485/RS422 Transceiver}

Texas Instruments 'TI' {RS-232/RS-422/RS-423/RS-422 Differential Driver/Receiver ICs}

RS485 Logic Diagram
RS485 Transceiver

Logic Functions;

Memory Chip IC Manufacturers
Processor Chip Manufacturers
Glue Logic IC Manufacturers

{EIA-485 Bus Index}

EIA485 Differential Bus Pull-up/Pull-Down Resistors

The Pullup/Down Resistors (Idle-line failsafe) are used to keep the +/- inputs of the Receiver(s) inputs at a minimum of 200mV differential.
Normally required once the system has reached it's quiescent state, when no drivers are driving the bus. Only one end of the bus requires the resistor network.

The resistor values are dictated by four factors: The input impedance of the receivers (between 5K and 90K ~ check the data sheet).
The number of loads, the termination resistance 60 ohms (120 ohms per side), and the required (turn-on) current of the receivers.
Many (on-line) App-notes use 12K ohms as an input impedance, 60 ohms (as a termination), 3.5 mA as the minimum 'load' current, and 10 loads.
Using these values {Check the data sheets for the real values!}:
..... Number of loads (10) x 12KW ~ 1200W {That's ten 12Kohm resistors in parallel, 12K//10 loads}
......Line Termination ~ 60 ohms {120 ohms at each side of the link, in parallel, 120//120}
......Taking 1200 in parallel with 60 ohms gives 57 ohms
......Turn-On current ~ 3.5 mA {check the data sheet for the device used}
The required load is 5 volts / 3.5mA, which results in a load impedance of (around) 1428 ohms. Subtract 57W which is already present leaves; 1371W.
Placing half on the +5 volts line and the other on the ground line, gives 675W per resistor / per supply.
The nearest 5% standard value is 680W placed between both the 'A' line and +V connection and the 'B' line and the GND connection will produce < 3.5 mA (turn-on current}

{RS485 Bus Index}

RS485 Cabling

RS422 and RS485 do not define cabling or connector information. RS449 and RS530 [listed below] are two common cabling specifications which are used with the RS422/485 electrical standards that do define the type of RS485 Cable/RS485 Connector. Use 24 AWG twisted pair copper wire with 16pF/ft for cables. Maximum cable length is determined by the combination of cable length (in meters) and data signaling rate (in bps). Charts of Cable Data Rate vs Cable Length. RS422 should not exceed 108 for length x bps. The example provided a maximum speed of 200kbps with a maximum cable length of 500 meter [500 x 200,000]. Refer to 24AWG Attenuation vs Frequency Chart.

EIA/TIA-449; General Purpose 37-Position Interface for Data terminal Equipment and Data Circuit-Terminating Equipment Employing Serial binary Data Interchange

A serial mechanical interface standard for transmission of balanced and unbalanced signals between a variety of higher-end computer, media, and multimedia peripherals.
EIA-449 allows a maximum data rate of 10 Mbit/s and uses a 37- or 9-pin connector.

EIA/TIA-530; High Speed 25-Position Interface for Data Terminal Equipment and Data Circuit-Terminating Equipment, Including Alternative 26-Position Connector

EIA-530 is a replacement for EIA-449 that uses a DB-25 (EIA-232-D) connector instead of a 37-pin connector, while keeping the most important electrical signals intact.
EIA-530 is to be used in conjunction with EIA-422-A.
RS-530, 25pin pinout, Signal assignments.
RS-449, 37pin pinout
RS-449, 9pin pinout

V.35/RS449 Max length
Data Rate Cable Length
2 Mb/sec 15.24 Meters
1 Mb/sec 30.48 Meters
512 Kb/sec 60.96 Meters
256 Kb/sec 121.92 Meters
128 Kb/sec 243.84 Meters
56 K 487.68 Meters
1.2 Kb/sec 914.40 Meters

RS422 pinout is not defined in the standard, any pinout table found for RS422 is non-standard.

{RS485 Bus Index}

OEM RS485 Interface Converters

RS485 Component Vendors:
Acces I/O Products Inc. {PCI/USB based RS485 Serial Communications Adapter}

ARC Electronics {RS-232 to RS-422/RS-232 to RS485 Converters}

B&B electronics {RS422 Converter/Optically Isolated RS485 Converter/RS232 to RS422 Converters}

EQUIP'TRANS {V11/V24/20mA to RS485 Converter Devices}

KK Systems Ltd. {RS232-RS422/RS485 Isolated Interface Converter, DIN Rail Products}

Lascomm {Converter EIA232 to RS485 Converters - RS232 to V.35/X21}

Patton Electronics Co. {RS-232/RS-422/RS-485 Converters}

Telebyte {RS-232 to RS-422/RS485 Interface Converters; Wiring Hub, DIN Rail Mounted}

{RS485 Bus Index}

Normally EIA422 and RS423 systems may not be connected together. EIA-449 cabling of RS422 sends and receives data as differential pairs and control signal as single-ended, but for 423 cabling it sends and receives single-ended data and control signals. Receiving the single ended signal in 423 is accomplished by grounding the 'B' side of the differential receiver at the connector.

So if the system follows the EIA-449 (cabling) specification than one (B) side of the differential receiver of the 423 side will be grounded at the connector forcing the differential driver on the 422 side to drive ground. The EIA422 side also uses a termination resistor between the ends of its differential receiver, providing a serious 120 ohm short to ground for the 423 driver.

If the systems were to be connected together (with out regard for RS499) than the system would revert to EIA423 (single-ended) distance and data rate ~ Only because the driver on the RS423 side is single ended, while the receiver on the RS422 side would receive the single-ended 423 signal and ground on its differential pair. How ever because 423 and 422 use the same receiver chip; going from 422 to 423 provides a differential path.

EIA422/423 Interconnection
RS422 to RS423 Conversion

EIA422/232 Interconnection
RS422 to RS232 Conversion

{Back to RS485 Interface Index}

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Modified: 2/26/12
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