We would like to confirm the Fail safe feature of ADM3485EARZ.
and have two questions as below
1 In the data sheet of ADM34858EARZ, it is with fail-safe feature.
But it do not present the voltage range of fail-safe.
We see AN-960(Implementation Guide) shows the fail-safe range of differential input threshold voltage
is -30mV to -200mV. Is ADM34858EARZ with the fail-safe range of differential input threshold voltage?
2. In AN-960, New generation RS-485 transceivers have an improved feature that includes true fail-safe receiver inputs.
This eliminates the need for pull-up/pull-down resistors.
Our original circuit is with 330 ohm pull high and pull low resistor(Let the differential voltage between A and B is 500mV when the differential output is open)
Due to the 330 ohm cause a Main chip boot problem. We would like to remove the 330 ohm pull high and pull low resistor.
Please help to confirm if ADM3485EARZ chip is with true fail-safe feature and removing 330 ohm is OK.
For question 1: The differential input threshold voltage is -200mV to 200mV, and ADM3485E only has open fail-safe feature rather than true fail safe.
question 2: It does not have true fail-safe so the bias resistor is needed.
Got it Thanks! We will add bias-resistor for open fail-safe feature.
The Board use two ADM3485E chip, we add 330 ohm bias-resistor as below two figures.
Fig.1 is with Terminal resistor. Fig.2 is without Terminal resistor.
Could you help to confirm if 330 ohm is okay for chip and RS-485 transmission?
Due to 330 ohm cause one system boot up issue.
We need to change 330 ohm to 5K ohm.
If we change 330 ohm to 5K, A-B is +39mV.
When Bus idle and A-B = 39mV, Can RO hold out 3.3V ?
The 39mV is not enough, and 900Ohm is recommended, would the 900 Ohm cause boot up problem? Do you have good bypass on power supply?
We do not have good bypass on power supply.
We use system 3.3V to pull high/low resistor of RS-485 A, B line
However when DUT connect with 3 RS-485 device.
Their pull high voltage will go through the system 3.3 V net
and cause that SOC chip power on reset issue.
Could you help to suggest the bypass ways on power supply?.
I suggest you to check the system 3.3V current output capability, and add some inductors on SOC chip supply net to improve the noise immunity, also large bypass capacitors may be needed.
We have two question as below
1. In original design, Pull high resistor is connected to VDD33(Main chip power)
We plan change to connect with 5V and series a DIODE(100V 200mA).
The purpose of DIODE is to block the DC current form the pull high circuit of other link RS-485 device.
We had rework board to confirm it work. Is there other points that we need to care on the DIODE?
2. The ADM3485E use VDD33(Main chip power). The pull high resistor connect with VCC5V.
In RS-485 transmission, should they use independent DC-DC ?
The Diode has large current rating which would be over design( the DC bias current is at several mA level) and you need take the diode voltage drops into consideration when calculating the resistors value, the bias resistors can limit the max. current well.
If the front stage DC-DC could output enough current at worse condition(inrush current/ extreme temperatures etc.), I don't think independent DC-DC is needed.