ADM3485E
STANDARDS AND TESTING
Table 6 compares RS-422 and RS-485 interface standards, and
Table 7 and Table 8 show transmitting and receiving truth tables.
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.
Table 6.
Specification
RS-422
Differential
10 Mbps
4000 ft
2 V
100 Ω
4 kΩ min
200 mV
RS-485
Differential
10 Mbps
4000 ft
1.5 V
54 Ω
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
destruction 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, which can
result in degraded performance. The cumulative effects of
continuous exposure can eventually lead to complete failure.
Transmission Type
Maximum Data Rate
Maximum Cable Length
Minimum Driver Output Voltage
Driver Load Impedance
Receiver Input Resistance
Receiver Input Sensitivity
Receiver Input Voltage Range
Number of Drivers/Receivers per Line
12 kΩ min
200 mV
−7 V to +7 V −7 V to +12 V
1/10 32/32
I/O lines are particularly vulnerable to ESD damage. Simply
touching or plugging in an I/O cable can result in a static
discharge that can damage or completely destroy the interface
product connected to the I/O port. It is extremely important,
therefore, to have high levels of ESD protection on the I/O lines.
Table 7. Transmitting Truth Table
Transmitting Inputs
Transmitting Outputs
A
RE
DE
DI
B
X1
X1
0
1
1
0
0
1
0
X1
X1
0
1
1
0
The ESD discharge could induce latch-up in the device under
test, so it is important that ESD testing on the I/O pins be
carried out while device power is applied. This type of testing is
more representative of a real-world I/O discharge, where the
equipment is operating normally when the discharge occurs.
High-Z2
High-Z2
High-Z2
High-Z2
1
1 X = don't care.
2 High-Z = high impedance.
Table 9. ESD Test Results
ESD Test Method
Table 8. Receiving Truth Table
I/O Pins
Receiving Inputs
Receiving Outputs
Human Body Model
15 kV
RE
DE
A – B RO
0
0
0
1
X1
X1
X1
X1
> +0.2 V
< –0.2 V
Inputs open
X1
1
0
1
100%
90%
High-Z2
1 X = don't care.
2 High-Z = high impedance.
ESD TESTING
36.8%
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 unit under test. With air-gap discharge, the discharge gun is
moved toward the unit under test, developing an arc across the
air gap, hence the term air-gap discharge. This method is
10%
TIME
t
tDL
tRL
Figure 24. Human Body Model Current Waveform
Rev. D | Page 11 of 16
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