THVD1450: Design RS485 auto-direction control circuit

Blindu Marcel

Part Number: THVD1450
Other Parts Discussed in Thread: NA555, THVD1426, THVD1406, TIDA-01090

Tool/software:

Hi,

We want to design a RS485 half duplex using only two pins (TX and RX). We see this TI Design using IC NA555 to control the data direction of the transceiver https://www.ti.com/tool/TIDA-01090 but we don`t know if are another method to design this part of circuit using this RS485 transceiver.

Best regards

1 month ago

0 Clemens Ladisch 1 month ago

Guru 304260 points

Just use the THVD1406/THVD1426.

0 Blindu Marcel 1 month ago in reply to Clemens Ladisch

Intellectual 325 points

We want to use a tipycal RS485 transceivers (without integrated auto direction control) because our client need to produce this product for more 10 years.

0 Clemens Ladisch 1 month ago in reply to Blindu Marcel

Guru 304260 points

Then you can use the method in TIDA-01090.

0 Blindu Marcel 1 month ago in reply to Clemens Ladisch

Intellectual 325 points

Ok we think that are other methods to do this auto direction control.

+1 Clemens Ladisch 1 month ago in reply to Blindu Marcel

Guru 304260 points

There are three methods:

Drive DE high after a falling edge on D for a certain time. This is what TIDA-01090 does.
Drive DE high after a rising edge on D for a certain time. This is what the THVD14x6 does. It would be harder to implement with discrete logic.
Drive DE high after a rising edge on D until the differential bus voltage reaches +200 mV. This is what the MAX1348xE does. It would be much harder to implement with discrete logic, and I would be surprised if this was not patented.

0 Blindu Marcel 1 month ago in reply to Clemens Ladisch

Intellectual 325 points

I see in one video this methods but i dont know if first ans third method are good or not:

0 BOBBY 1 month ago in reply to Blindu Marcel

TI__Guru 65390 points

Controlling the DE pin has some disadvantages since these devices don't generally have the same enable and disable times so if the data rate you need has to be fast it can cause bit errors due to the bits getting stretched during the enable/disable. Generally I see customers who control DE directly have issues when running at 115kbps but it works fine at 9600 bps.

-Bobby

+1 Clemens Ladisch 1 month ago in reply to Blindu Marcel

Guru 304260 points

These are similar to the second method, but the switching thresholds are not very well defined, so the time delay is somewhat random.

(And the "Transistor" circuit lacks the Schmitt-trigger buffer, and I cannot see how the transistor would be an improvement over the diode.)

Interface

Interface forum

THVD1450: Design RS485 auto-direction control circuit