THVD1550: Is there any problem with turning on the driver and receiver at the same time

Hi Clemens,

Thank you for your reply.

Could you please let me confirm regarding your answer.

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1. You are mentioning the Driver Enable time and Receiver Enable time described in 7.8 of the data sheet.

Regarding the Driver Enable time,

when RE: 0V (enable), tpZH is 10nstyp.

When RE: Vcc (disable), tpZH is 2μs.

Regarding the Receiver Enable time,

when DE = Vcc (enable), tpZH is 55nstyp.

when DE = 0V (disable), tpZH is 3μstyp.

I think this implies that the fact that it takes time to recover means that it has entered standby.

However, I think it is difficult to understand the standby state from this description alone.

Are there any documents that clearly state this?

2. I understand.

3. When transmitting and receiving at the same time, the driver output of one transceiver will be output to the receiver.

Is it possible to assume that there is no risk of damaging the THVD1550 and that it will not affect communication with the outside?

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I appreciate your great help and cooperation.

Best regards,

Shinichi

Hi Shinichi,

I believe Clemens has answered your questions but to summarize, the 'standby' mode is simply the device in it's lowest powered state used to conserve power by disabling the receiver and transmitter function. (We don't really have any documents to provide on the topic of 'standby'). When the receiver is disabled the R output is high impedance (undefined) unless you have an external resistor tied to Vcc or GND. It takes a bit of time before the receiver is turned on and senses the input state then turns on the Routput. Enabling/disabling both at the same time is NOT expected to damage the device.

Thanks Clemens for providing your inputs as usual.

-Bobby

EDIT: fixed typo of Enable/disable NOT expected to damage device.

Hi Bobby,

Thank you for your comment,

I'd like to confirm.

You mentioned that enabling/disabling the device at the same time could damage the device.

Doesn't this conflict with what Clemens recommended - turning on the driver/receiver at the same time would prevent the device from going into standby?

I appreciate your great help and cooperation.

Best regards,

Shinichi

Hi Shinichi,

Very sorry about that, I meant to say it's not expected to damage the device. A typo on my part. Clemen's statement is correct. I've edited my response to fix my typo.

-Bobby

HI Bobby,

Thank you for your reply.

I understand.

I'll share with the customer that enabling/disabling the device at the same time could not damage the device.

I appreciate your great help and cooperation.

When the customer has an additional question, I consult you again.

Best regards,

Shinichi

Hi Bobby,

Thank you for your reply.

The customer has an additional question regarding question(3).

Could you please advise?

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I would like to confirm additional question(3).  

The circuit in Figure 28 of page 22 of the datasheet: 10.2.2 controls RE and DE simultaneously with the DIR pin of the MCU.

We are using this circuit in another product.

I think there are two states:

① RE=Disable, DE=Enable

② RE=Enable, DE=Disable

In this case, RE and DE are never enabled or disabled at the same time

When switching from state ① to state ②, does this mean that the R output buffer will not become an undefined output?

Since it is a simultaneous control, it is not in standby mode, so I assume that it will not become an undefined output.

Actually, it did not appear that there was any incorrect output on the actual device.

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I appreciate your great help and cooperation.

Best regards,

Shinichi

Hi Bobby,

Thank you for your reply.

I'll share your answer with the customer.

When the customer has an additional question, I consult you again.

I appreciate your great help and cooperation.

Best regards,

Shinichi

Sure thing Shinichi, Feel free to reply back to this thread if it's still open or create a new thread.

-Bobby

Hi Bobby,

The customer has an additional question.

Could you please advise?

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In the case of the circuit in Figure 8, the 90ns or more interval from /RE (Enable) to DE (Disable) cannot be maintained (attached PPT).

What happens to the R output of the THVD1550 immediately after switching from ① to ② by simultaneous control?

① RE=Disable, DE=Enable

② RE=Enable, DE=Disable

As in the previous answer,

> ⇒The RS-485 receiving circuit takes longer to power up than the R output pin buffer, so the R pin outputs a signal that does not correspond to the bus state.

Or,

・It does not go into standby mode, so it outputs the most recent D output (= bus state), etc.

/cfs-file/__key/communityserver-discussions-components-files/138/THVD1550-DE-RE-Control-Timing_0030_.pdf

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I appreciate your great help and cooperation.

Best regards,

Shinichi

Shinichi,

Please expect delays as it is a US holiday this week. We will get back to you with a response by 07/09/2024.

Regards,

Eric Hackett

Shinichi Inoue said:

In the case of the circuit in Figure 8, the 90ns or more interval from /RE (Enable) to DE (Disable) cannot be maintained (attached PPT).

What happens to the R output of the THVD1550 immediately after switching from ① to ② by simultaneous control?

① RE=Disable, DE=Enable

② RE=Enable, DE=Disable

As in the previous answer,

> ⇒The RS-485 receiving circuit takes longer to power up than the R output pin buffer, so the R pin outputs a signal that does not correspond to the bus state.

I'm going to rephrase your question since you are stating 90ns or more.... What happens when the receiver is enabled but the driver is disabled?

If you have external fail safe resistors then the output of R should be high. (I assume the A-B is a positive voltage in the external fail safe)

If you do not have external fail safe and no other devices on the bus are driving the bus line, then the receiver will see a floating input. It is likely the receiver will see the signal of A-B as positive and since the device has an internal fail safe (Vth+ is negative) then the R output should be high. If noise couples onto the receiver input pins and brings A-B negative then it is possible for the Routput to change to a logic low.

If the last bit you sent on the D input pin was a 0 and you went to disable the driver, the bus RS485 bus would slowly discharge it's parasitic caps and A-B would likely go from a negative value to a positive value. So R would likely see a low first then transition back high when A-B becomes positive.

This is described in table 2 of the datasheet.

What happens if the transition is much faster (less) than 90ns?

You only have a risk of entering standby if your delay time between transitioning from 1 to 2 is below 90ns. In the situation if you do enter standby, you should exit standby 8uS later. During the standby phase, the R output pin would be high-Z.

-Bobby

Hi bobby,

Thank you for your reply.

The text was inappropriate. I'm sorry.

I'd like to say, In the case of the circuit in Figure 8 of the THVD1550 data sheet, /RE (Enable) to DE (Disable) are controlled simultaneously, and it is less than 90ns.

In this case, is the comment you mentioned the answer?

BOBBY said:

You only have a risk of entering standby if your delay time between transitioning from 1 to 2 is below 90ns. In the situation if you do enter standby, you should exit standby 8uS later. During the standby phase, the R output pin would be high-Z.

In other words, is it correct to think that the circuit in Figure 8 may enter standby mode when switching between transmit and receive?

In the customer's actual operation, it does not enter standby mode.

I appreciate your great help and cooperation.

Best regards,

Shinichi

Shinichi Inoue said:

In other words, is it correct to think that the circuit in Figure 8 may enter standby mode when switching between transmit and receive?

In theory it would be possible for you to enter standby mode when both gets switched at the exact same time since the driver should disable before the receiver is enabled.

It's pretty common for customers to tie the RE and DE pins together based on customer schematic reviews I've seen. In practice I don't think I've heard of a customer enter this mode and report it before. You would probably be able to tell if it occurred where the Routput would become high-z and float from high to low very slowly. Adding a weak external pull down resistor (like 100k) on the pin would show a slow RC decay.

-Bobby