ADM3485E
Rev. D | Page 11 of 16
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.
Table 6.
Specification RS-422 RS-485
Transmission Type Differential Differential
Maximum Data Rate 10 Mbps 10 Mbps
Maximum Cable Length 4000 ft 4000 ft
Minimum Driver Output Voltage ±2 V ±1.5 V
Driver Load Impedance 100 Ω 54 Ω
Receiver Input Resistance 4 kΩ min 12 kΩ min
Receiver Input Sensitivity ±200 mV ±200 mV
Receiver Input Voltage Range −7 V to +7 V −7 V to +12 V
Number of Drivers/Receivers per Line 1/10 32/32
Table 7. Transmitting Truth Table
Transmitting Inputs Transmitting Outputs
RE
DE DI B A
X
1
1 1 0 1
X
1
1 0 1 0
0 0 X
1
High-Z
2
High-Z
2
1 0 X
1
High-Z
2
High-Z
2
1
X = don't care.
2
High-Z = high impedance.
Table 8. Receiving Truth Table
Receiving Inputs Receiving Outputs
RE
DE A – B RO
0 X
1
> +0.2 V 1
0 X
1
< –0.2 V 0
0 X
1
Inputs open 1
1 X
1
X
1
High-Z
2
1
X = don't care.
2
High-Z = high impedance.
ESD TESTING
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
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
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.
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.
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.
Table 9. ESD Test Results
ESD Test Method I/O Pins
Human Body Model ±15 kV
100%
90%
36.8%
10%
t
RL
t
DL
I
PEAK
TIME
t
03338-023
Figure 24. Human Body Model Current Waveform
