Date: 6/21/04

SP485R RS-485 Serial Transceiver

© Copyright 2004 Sipex Corporation

6

with the two 125

Ω cable termination resistors

on each end.

The receiver contains a enable pin (RE) which

enables the receiver when a logic LOW is

asserted. A logic HIGH will tri-state the

receiver output and the inputs will maintain

at least 120k

Ω impedance. The receiver can

operate to at least 1Mbps.

The receiver also contains a fail-safe feature

which outputs a logic HIGH when the inputs are

open as in a disconnected cable.

SHUTDOWN MODE

The SP481R includes a shutdown function to

reduce power consumption. The shutdown is

activated by simultaneously applying a logic

LOW to DE and a logic HIGH to RE. While in

the shutdown mode, the power supply current is

typically less than 1

µA. The driver outputs are

disabled and are at a high impedance state

determined by the receiver input impedance

which should be at least 120k

Ω. The receiver

output is at also at high impedance during

shutdow. Output leakage current when the

receiver is disabled is under 1

µA.

GENERAL DESCRIPTION

The SP485R is a low power RS-485 differential

transceiver. Similar to the SP485, the SP485R

contains a half-duplex driver and receiver with

tri-state control. However, the SP485R is in-

tended for increased connections on a single bus

compared to the orignal RS-485 specification.

The RS-485 standard is ideal for multi-drop

applications where one bus can contain many

drivers and/or receivers. The RS-485 standard

implementation allows up to 32 transceivers to

be connected on to the data bus. RS-485 is also

specified for driving higher speeds over long

cable lengths of up to 4,000 feet. The SP485R

and SP481R exceed the standard b allowing up

to 400 receivers to share a bus.

DRIVERS

The driver output complies with the RS-485

electrical characteristics as specified by the

standard. The output swings from 0V to V

maintains greater than +1.5V with a 54

Ω load

attached between the two outputs. In adhering

to the RS-485 specification, the driver outputs

inherently comply with the RS-422 standard.

With a load of 100

Ω between the two outputs,

the driver can sustain at least +2.0V.

The driver contains an enable pin (DE) which

tri-states the output when DE is a logic LOW.

The outputs during the tri-stated condition are

at a high impedance (>100k

Ω). A logic HIGH

enables the driver for normal operation. The

driver can operate to at least 5Mbps.

RECEIVERS

The SP485R receiver has differential inputs

with an input sensitivity of lower than

±200mV.

As mentioned above, the RS-485 specification

allows up to 32 transceivers on a the same bus.

The SP485R allows over 400 transceivers on

the same bus due to the high input impedance of

at least 120k

Ω. This higher capacity allows

more components to be attached to the same

bus without degrading the signal quality. The

drivers are still able to drive an equivalent 54

Ω

from the 320 transceivers with an input

impedance of at least 120k

Ω in parallel along