Datasheet

ADM3070E/ADM3071E/ADM3072E/ADM3073E/ADM3074E/ADM3075E/ADM3076E/ADM3077E/ADM3078E
Rev. E | Page 16 of 20
HOT-SWAP CAPABILITY
(ALL EXCEPT ADM3071E/ADM3074E/ADM3077E)
Hot-Swap Inputs
When a circuit board is inserted into a hot (or powered) back-
plane, differential disturbances to the data bus can lead to data
errors. During this period, processor logic output drivers are
high impedance and are unable to drive the DE and
RE
inputs
of the RS-485 transceivers to a defined logic level. Leakage currents
up to ±10 μA from the high impedance state of the processor
logic drivers can cause standard CMOS enable inputs of a tran-
sceiver to drift to an incorrect logic level. Additionally, parasitic
circuit board capacitance can cause coupling of V
CC
or GND to
the enable inputs. Without the hot-swap capability, these factors
can improperly enable the driver or receiver of the transceiver.
When V
CC
rises, an internal pull-down circuit holds DE low and
RE
high. After the initial power-up sequence, the pull-down
circuit becomes transparent, resetting the hot-swap tolerable input.
LINE LENGTH vs. DATA RATE
The RS-485/RS-422 standard covers line lengths up to 4000 feet.
For line lengths greater than 4000 feet, Figure 37 illustrates an
example line repeater.
±15 kV ESD PROTECTION
Two coupling methods are used for ESD testing: contact
discharge and air-gap discharge. Contact discharge calls for
a direct connection to the unit being tested. Air-gap discharge
uses a higher test voltage but does not make direct contact with
the test unit. With air-gap discharge, the discharge gun is moved
toward the unit under test, developing an arc across the air gap,
thus the term air-gap discharge. This method is influenced by
humidity, temperature, barometric pressure, distance, and rate
of closure of the discharge gun. The contact discharge method,
while less realistic, is more repeatable and is gaining acceptance
and preference over the air-gap method.
Although very little energy is contained within an ESD pulse, the
extremely fast rise time, coupled with high voltages, can cause
failures in unprotected semiconductors. Catastrophic destruc-
tion can occur immediately as a result of arcing or heating.
Even if catastrophic failure does not occur immediately, the
device can suffer from parametric degradation that can result
in degraded performance. The cumulative effects of continuous
exposure can eventually lead to complete failure.
Input/output lines are particularly vulnerable to ESD damage.
Simply touching or connecting an input/output cable can result
in a static discharge that damages or completely destroys the
interface product connected to the input/output port. It is
extremely important, therefore, to have high levels of ESD
protection on the input/output lines.
The ESD discharge can induce latch-up in the device under test,
so it is important that ESD testing on the input/output pins be
carried out while device power is applied. This type of testing
is more representative of a real-world input/output discharge,
which occurs when equipment is operating normally.
The transmitter outputs and receiver inputs of the ADM307xE
family are characterized for protection to a ±15 kV limit using
the human body model.
HUMAN BODY MODEL
Figure 33 shows the human body model and the current
waveform it generates when discharged into low impedance.
This model consists of a 100 pF capacitor charged to the ESD
voltage of interest, which is then discharged into the test device
through a 1.5 kΩ resistor.
100%
90%
36.8
%
10%
t
RL
t
DL
I
PEAK
TIME t
R2
C1
R1
HIGH
VOLTAGE
GENERATOR
DEVICE
UNDER
TEST
ESD TEST METHOD
HUMAN BODY MODEL
ESD ASSOC. STD 55.1
R2
1.5k
C1
100pF
06285-015
Figure 33. Human Body Model and Current Waveform
256 TRANSCEIVERS ON THE BUS
The standard RS-485 receiver input impedance is 12 kΩ (1 unit
load), and the standard driver can drive up to 32 unit loads. The
ADM307xE family of transceivers has a ⅛ unit load receiver
input impedance (96 kΩ), allowing up to 256 transceivers to be
connected in parallel on one communication line. Any combi-
nation of these devices and other RS-485 transceivers with a
total of 32 unit loads or fewer can be connected to the line.
REDUCED EMI AND REFLECTIONS
The ADM3070E/ADM3071E/ADM3072E feature reduced
slew rate drivers that minimize EMI and reduce reflections
caused by improperly terminated cables, allowing for error-
free data transmission at rates up to 250 kbps. The ADM3073E/
ADM3074E/ADM3075E offer higher driver output slew rate
limits, allowing for transmit speeds of up to 500 kbps.