TIA/EIA-485 Electrical Characteristics of Generators and Receivers for
Use in Balanced Digital Multipoint Systems
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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/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 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 [www.tiaonline.org]
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 [www.itu.int]
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 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 Transceiver |
Logic Functions; |
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}
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
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 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}
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}
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