3 EVM Setup and Precautions

Figure 3-1 shows the schematic of the EVM. The EVM board has headers labeled from JMP1 to JMP14 (JMP5 is omitted) and two 3-pin terminal blocks labeled TB1 and TB2. These headers support device evaluation for a wide range of system configurations.

• Pin 1 (EARTH) is a second ground pin that allows applying an external voltage between GND and EARTH to simulate common-mode voltage conditions.
• Pin 2 (GND) is connected to the negative output or ground terminal of the PSU. This pin represents the ground potential of the device-under-test and the entire EVM. It also connects to various jumpers on the board.
• Pin 3 (VCC) is connected to the positive output of a regulated power supply unit (PSU) as it represents the positive supply voltage of the device-under-test and also connects to various jumpers on the board.

Figure 3-1 RS-485 Half-Duplex EVM Schematic

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 3-2 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_CC requirements of the RS-485 device.

Figure 3-3 Example for Stimulus and Probe Points with JMP4 and JMP14

Figure 3-3 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_CC 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_IH ≥ 2 V and the low-level input voltage V_IL ≤ 0.8 V.