Datasheet
Vcc
GND
B
A
D
R
1
2
4 5
6
7
8
DE
3
U1
RE
DUT_VCC
R5
60 W
0V
3.3V
Receive
Data
B
A
Transmit
Data
V
OD
375 W
R8
375 W
R9
V
CM
= -7V to +12V
Scope
Ch1 Ch2 Ch3 Ch4
PSU1
3.3V
Signal
Generator
JMP4
1
2
VCC
3
4
R
D
JMP3
1
2
VCC
3
4
JMP2
1
2
VCC
3
4
/RE
DE
JMP6
1
2
3
4
B
A
1
JMP11
2
3
1
JMP14
2
3
1
2
3
TB1
GND
EARTH
VCC
3.3V
Powering Up the EVM and Taking Measurements
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Figure 6. RS-485 Half-Duplex EVM Setup for Normal Transceiver Operation
The low potential for RE is provided by the wire-bridge from pin 2 to pin 3 at JMP2, and the high
potential for DE through a wire-bridge from pin 2 to pin 1 at JMP3. Data from the signal generator enter
the board at pin 2 and pin 3 of JMP4. This data is measured via channel 1, which is connected to pin 1
and pin 2 of JMP14. Channel 2 measures the receive data at JMP11, and channels 3 and 4 the bus
voltages, V
A
and V
B
, at JMP6.
2. Operation Under Maximum Load
EIA-485 (RS-485) specifies three maximum load parameters: a maximum differential load of 60 Ω, a
maximum common-mode load of 375 Ω for each bus wire, and a receiver common-mode voltage
range from –7 V to +12 V. Figure 7 reflects these requirements through R5, R8, R9, and V
CM
. Note that
under maximum load conditions the transceiver must be capable of sourcing and sinking bus currents
of up to 55 mA. The purpose of this test is to show the robustness of V
OD
over the entire common-
mode voltage range at maximum load.
Figure 7. Configuration for Maximum Loading
While the cable connections of the signal generator and the oscilloscope remain the same as in the
previous example, the following board changes need to be implemented to reflect maximum load
conditions:
6
RS-485 Half-Duplex Evaluation Module SLLU173B–October 2012–Revised June 2013
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