Each of the following measurement examples show the equivalent circuit diagram and the corresponding EVM setup. Only the measurement relevant headers and terminal blocks are shown, and not necessarily at their exact location on the EVM.

1. Standard Transceiver Configuration

   Normal transceiver operation requires both the driver and the receiver sections being active. Therefore, the receiver enable pin ( RE) must be at logic low potential and the driver enable pin (DE) at logic high.

   Transmit data entering at the D-input terminal appear as the differential output voltage (V_OD = V_A – V_B) on the bus wires, A and B. Via the active receiver, it is possible to sense the data traffic in transmit direction.

   Figure 4-1 Transceiver Configuration for Normal Operation

   Figure 4-2 shows the corresponding EVM setup. EARTH and GND receive the same reference potential, PSU-ground, through the wire-bridge from pin 1 to pin 2 at the terminal block, TB1, while pin 3 (VCC), in this example, is connected to the 3.3-V output of a power-supply unit (PSU).

   Figure 4-2 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 4-3 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 4-3 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:

   • replace R5 (120-Ω default) with 60 Ω
   • replace R8 and R9 (0-Ω default) with 375 Ω
   • connect pin 2 of JMP7 with pin 1 and pin 3 with pin 4
   • replace the previous wire-bridge at TB1 with a second power supply unit (PSU2) and connect the ground terminals of both, PSU1 and PSU2 with a wire-bridge, as shown in Figure 4-4.
   Figure 4-4 RS-485 Half-Duplex EVM Setup for Maximum Loading

   Note that Figure 4-4 only shows the wiring of PSU2 for positive common-mode voltages. For negative V_CM, connect the ground terminal of PSU2 with pin 1 of TB1 (EARTH), and the V_CM-output of PSU2 with the ground terminal of PSU1.

   Figure 4-5 RS-485 Half-Duplex EVM Configurations: Left as Receiver EVM, Right as Transmitter EVM

Figure 4-6 Top View of RS-485 Half-Duplex EVM

Figure 4-7 Bottom View of RS-485 Half-Duplex EVM

For detailed information on the device parameters see the data sheet of the selected device at www.ti.com