Datasheet

3.3V

Signal

Generator

Scope

Ch1 Ch2

JMP4

VCC

JMP14

D - input

3.3V

PSU

TB1

EVM Setup and Precautions

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For the first measurements, ignore the common-mode simulation and connect EARTH to GND through a

wire-bridge between pin 1 and pin 2 of TB1.

Figure 3. Bridging DUT_GND with EARTH_GND

While JMP2 to JMP4 are stimulation points, or headers through which the control and data signals for the

RS485 Half-Duplex EVM are applied, JMP1, and JMP11 to JMP14 are probe points, or headers at which

these signal can be measured.

Note that the 50-Ω resistors, R2, R3, and R4, have the index n.a., indicating that these components are

not assembled. Because signal generators have a typical source impedance of 50 Ω, their output signal is

twice the required signal voltage, and assumes that the on-board 50-Ω resistors divide this voltage down

to the correct signal level.

Without these resistors; however, this voltage divider action is not accomplished, and the generator output

voltage must be reduced to match the V

requirements of the RS-485 device.

Figure 4. Example for Stimulus and Probe Points with JMP4 and JMP14

Figure 4 gives an example for entering a data signal into the driver section of the transceiver. The signal

output of the generator is adjusted to match the device V

power supply requirements . The generator’s

ground terminal is connected with pin 3, and the signal output terminal with pin 2 of JMP4. The data signal

is measured through an oscilloscope with its signal input connected to pin 1 and its ground wire connected

to pin 2 of JMP14.

The same setup applies to the DE and RE inputs through their corresponding headers JMP2 and JMP12

and JMP3 and JMP13. JMP1 however, must not receive a signal stimulus. Like JMP11, it represents the

receiver output, R, of the half-duplex RS-485 device.

Instead of using signal generators, the EVM can directly interface to the micro controller I/O. Then the

non-assembled 50-Ω resistors are of no concern. However, for proper operation, it must be assured that

the high-level input voltage V

≥ 2 V and the low-level input voltage V

≤ 0.8 V.

RS-485 Half-Duplex Evaluation Module SLLU173B–October 2012–Revised June 2013

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