SLLA574 March   2022 THVD1406 , THVD1426

 

  1.   Trademarks
  2. 1Enable Pins and Auto-Direction
  3. 2Test Setup
    1. 2.1 Test One: Auto Direction in Action
    2. 2.2 Test Two: Sending Data Below Recommended Operating Conditions
    3. 2.3 Test Three: A System With No Termination
    4. 2.4 Test Four: A System With a Fully Loaded Bus
  4. 3Layout and Package Options
  5. 4Summary
  6. 5References

1 Enable Pins and Auto-Direction

The RS-485 interface works using differential signaling that is effective in rejecting common-mode noise making it a popular choice for long-distance communication. In applications, the enable pins of RS-485 transceivers are usually connected to general purpose input output (GPIO) of microcontrollers. For example, in a common 8-pin half-duplex RS-485 transceiver shown in Figure 1-1, the driver enable (active high) is pin 3 (DE), while the receiver enable (active low) is pin 2 (REB). The auto direction feature reduces the reliance on separate pins for driver-enable and the receiver-enable functionality.

Figure 1-1 Pinout of 8-pin RS-485 Transceiver

In applications, regular RS-485 drivers or receivers can be controlled by a microcontroller separately with 2 GPIO pins or with 1 GPIO pin if the DE pin and REB pin are shortened. Similarly, if a digital isolator is used for these logic controls, two isolation channels are needed for the DE and REB pin or one isolation channel is needed for the combined DE and REB control pins. To further reduce the number of GPIO pins for logic control or the number of isolation channels needed, the THVD14x6 (THVD1406 and THVD1426) devices can be used. These parts are half-duplex RS-485 transceivers with the auto direction control feature which do not include enable pins like DE and REB as shown in Figure 1-2. Instead, these devices use the data input pin (D pin) to initiate communication. Logic control happens automatically meaning software to support the enable controls are no longer required.

Figure 1-2 Pinout of THVD14x6

Installed in a system and powered on, THVD14x6 enters the receiver mode by default. When an input signal at the D pin (pin 4) toggles from high to low, the driver is triggered to be active and starts sending data. In RS-485 communication systems, universal asynchronous receiver-transmitter (UART) protocol is widely used. In this protocol, all data is transferred bit by bit, while the last bit is a stop bit which is normally high and the line idles as a logic high afterwords. As the THVD14x6’s input pin is pulled high for a certain time (tdevice_autodir), the driver becomes disabled and releases the bus. The driver active time is 0.8-μs in THVD1426 and 8-μs in THVD1406.

In this application note, several test cases are presented with various system configurations. The information presented can help a system designer understand how the auto direction feature of half duplex RS-485 devices work and aid to build successful systems with devices such as THVD14x6.