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]
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EIA-422/485 Bus Description

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.

{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

Electronic Industries Alliance {EIA}

Telecommunications Industry
Association {TIA}

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

ITU {International Telecommunication Union}

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

Dallas Semiconductor Corp "Maxim"
{RS422 / RS485 Line Driver/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}

SIPEX {RS-485 Differential
Driver/Receiver IC Manufacturer}

STMicroelectronics {RS-485/RS422 Transceiver}

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

Memory Chip IC Manufacturers
{This Site}
Processor Chip
Manufacturers {This Site}
Glue Logic IC
Manufacturers {This Site}

{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 5volts / 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). 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 (This web Site)
RS-449,
37pin pinout (This web Site)
RS-449, 9pin
pinout (This web Site)

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

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.

{Back to
RS485 Interface Index}

Engineering Design Key words: EIA-485, TIA-485, EIA422, TIA422, RS485,
RS422, Balanced differential Cable Bus, Electrical Standard, Interface Standard,
Specification, Spec, Circuit layout, Electrical Interface, Semiconductor IC
manufacturers, Physical Interface, Engineering Description, electrical
characteristics, Device, Component Manufacturers

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Last Modified 1/27/07
Copyright © 1998 - 2007 All rights reserved
Leroy Davis