EIA/TIA-485 Interface Circuit {RS485}
Just how close can two transceivers be placed on a bus. The maximum bus distance for any interface well know and dictated by ICs and interface medium. Bus what is the minimum distance?
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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
How close can I place the Bus Nodes.
This page provides two examples of
RS485 fault protection.
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
EIA; Electronic Industries Alliance
TIA; Telecommunications Industry
Association
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.
Manufacturers of Memory Chip ICs , Manufacturers of Processor Chips , Manufacturers of Glue Logic IC
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}
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 , RS-449, 37pin pinout , RS-449, 9pin pinout
| 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 |
{Back to RS485 Interface Index}
Engineering Design Key words: EIA-485, TIA-485, Node, 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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