Overview

The SP3070-3078E and SP3080-3088E families of advanced RS-485 transceivers contain a rich

feature set designed to simplify the design of robust, high performance RS-485 communication net-

works. All members of these two new product families remain fully backward compatible to industry

standard RS-485 devices, keeping the same footprint and pin locations as earlier products. They are

ideal for new designs or as replacements in existing RS-485 or RS-422 systems.

This document describes some of the new features found on SP3070-3078E and SP3080-3088E de-

vices and is intended to help system designers understand how to use these devices most effectively.

General Description

The SP3070E-3078E and SP3080-3088E families include a total of 8 products. SP3070-3078E op-

erate from a 3.3V supply voltage and SP3080-3088E operate from 5V supplies. Each family offers a

choice of slow, medium or fast data-rates and either half-duplex, full-duplex or full-duplex with enable/

shutdown. Each device contains one differential driver and one differential receiver. All comply with

both RS-485 and RS-422 EIA Standards.

Advanced Features of the SP3070E-SP3078E and SP3080E- SP3088E High Performance

RS-485 Transceivers

Application Note ANI20

3.0 to 3.6V Supply

Type

Pins

250kbps,

Low EMI

500kbps,

Reduced EMI

6Mbps,

High Speed

Half-duplex

SP3072E

SP3075E

SP3078E

Full-duplex,

Low Pin-count

SP307E

SP3074E

SP3077E

Full-duplex,

Enable/Shutdown

SP3070E

SP3073E

SP3076E

4.5 to 5.5V Supply

Type

Pins

5kbps,

Low EMI

500kbps,

Reduced EMI

20Mbps,

High Speed

Half-duplex

SP3082E

SP3085E

SP3088E

Full-duplex,

Low Pin-count

SP308E

SP3084E

SP3087E

Full-duplex,

Enable/Shutdown

SP3080E

SP3083E

SP3086E

Tables and 2 - Product Selection Matrices

Solved by

Significant Features of the SP3070E-3078E and SP3080E-3088E Families

The SP3070E-3078E and SP3080-3088E families share a number of enhanced features that differen-

tiate these products from other RS-485 transceivers. Below is a list of the "Top Ten" major enhance-

ments that separate these new products from ordinary RS-485 devices.

)

/8th Unit Load

Available as half-duplex, full duplex or full-duplex with enable/shutdown pins

Advanced Failsafe

Voltage Overload Protection

Thermal Overload Protection

Hot Swap and Glitch Protection

Slew Rate Controlled Driver

Low Power Shutdown

Enhanced ESD Protection

0) Industry Standard Package Footprints

1/8th Unit Load

All devices in the SP3070E-3078E and SP3080E-3088E families are /8th Unit Load transceivers.

This feature enables large RS-485 networks of up to 256 devices to operate over the full common

mode voltage range.

RS-485 is defined to allow multiple transmitters to coexist on a single segment of twisted-pair cable.

Only one transmitter may be active and driving at a time but many receivers or transceivers (with

driver off) may coexist. Receivers have high input impedance but will absorb some signal-current

from the bus. Each additional receiver or transceiver will therefore increase the loading on the driv-

ers. The number of nodes that any segment can support is therefore limited by the strength of the

drivers and the load characteristics of each receiver.

A standard RS-485 driver is capable of driving up to 32 "Unit Loads" plus two line-termination resis-

tors of 120 each. The EIA-485 standard defines Unit Load in terms of the current load over a com-

mon mode voltage range of -7V to +2V from device ground. Common mode voltage is an important

factor because at common mode further from device ground the driver must source or sink more cur-

rent into the load to maintain the same signal margin at a receiver.

The figure below is a simplified electrical model of a driver and load. Each terminal has an input im-

pedance (RA, RB) to local ground. Local ground at the load may be offset from the driver's reference

ground by some common mode voltage VOC. The driver attempts to maintain a differential voltage

VOD across the receiver inputs. Driver current into A and B increases proportionally to the magnitude

of VOC.

The dark shaded region in the figure below defines an RS-485 Unit Load. At +12V the current into

the load cannot exceed mA. At -7V current cannot exceed -0.8mA. A purely resistive load of

12,000 fits within this region. The actual load area extends along a 15k slope to points at -3V and

+5V, allowing for some non-linearity in the actual load.

12V

-3V

-7V

1mA

-0.8mA

RS485

1 Unit Load

(shaded area)

0.125mA

-0.1mA

1/8th Unit Load

(cross-hatched)

SP3070E-3078E and SP3080E-3088E devices are /8th Unit Load. This means that over the same

input voltage range they will load a driver only /8th as much. That area is shown in the cross-

hatched area of the figure above. At +12V input the current into a 1/8th UL device cannot exceed

25uA.

Typical RS-485 receivers are one Unit Load apiece. An RS-485 driver can maintain signal to 32 unit

loads over the -7V to +2V common mode range. With /8th Unit Load devices the same driver can

support eight times the number of transceivers, up to 256.

Different types of receivers may be mixed in a network. For every Unit Load transceiver you can

substitute eight /8th UL devices. This may extend the span of existing networks.

RS-422 or V. receivers may interoperate with RS-485. However they do not support the full com-

mon mode voltage range. Also they may have a lower input impedance of 4k, meaning that each

V. or RS-422 receiver consumes approximately 3 Unit Loads.

If the common mode voltage is limited, such as by bonding the grounds of all RS-485 nodes, then it

may be possible to build very large networks of hundreds of nodes. Using /8th UL devices will fur-

ther increase the number of transceivers.

Full-duplex or Half-duplex operation

The SP3070E-3078E and SP3080E-3088E families include devices in half-duplex, full duplex and

full-duplex with control inputs. All devices are available in industry standard packages and pinouts.

This makes Sipex enhanced SP3070 and SP3080 series devices ideal upgrades for existing RS-485

designs.

Half-Duplex is the most commonly used bus scheme for RS-485. Figure 2a shows a typical RS-485

Half Duplex data bus. As few as two or up to 32 Unit Loads, may coexist on the bus. Any node may

transmit but only one node may be driving at a time. Data travels bidirectionally on the bus.

The most significant advantage of Half-Duplex is that it requires only a single twisted-pair. This mini-

mizes cable costs. However because only one node at a time is permitted to transmit, half-duplex

operation requires bus arbitration to prevent data collisions. The RS-485 standard defines only the

electrical characteristics of the interface and does not specify arbitration techniques. Bus arbitration

is left up to the end-user. Various proprietary and open-standard interface protocols use RS-485 as a

physical media and define methodologies for bus arbitration.

Sipex SP3072E, SP3075E, SP3078E, SP3082E, SP3085E and SP3088E are configured for Half-Du-

plex operation. On these devices the data-bus pins A and B are bi-directional. Driver output enable

pin (active high) and receiver enable pin (active low) are opposite polarity and facilitate driver/receiver

control on a bidirectional data bus.

Full-Duplex (figure 2b) requires two twisted pairs.

On each wire-pair, data travels only in a single direction. The driver and receiver of each node are

physically connected to different wire-pairs. On each pair only a single driver may be active at any

given time.

This increases cabling costs but decreases complexity. Full-duplex may be configured point to point

(one driver into one receiver) or point to multipoint (one driver and many receivers). In a

: RS485 and RS422 physical topologies are explained in the Application Note ANI13. http://www.sipex.com/files/ApplicationNotes/

RS485%20Topology.pdf

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