RS-485 (MAX487 / MAX485) - On-chip fail-safe feature

[esm.]

Hello all.

I have a good knowledge on RS-485 buses. I have some doubts about the internal fail-safe feature of MAX487.

I know how to do a fail-safe biasing using external pull-up and pull-down resistors, but my doubt is just about the internal fail-safe feature of these transceivers.

Currently I can't do tests in practice with my ICs (MAX487). I won't use a MAX485 because I could need more than 32 nodes attached to the RS485 bus (MAX487 supports up to 128 nodes).

When receiving at the RS485 bus, I know that:
VA-VB >= 200mV ---> RO pin is level '1'
VA-VB <= 200mV ---> RO pin is level '0'
VA-VB > 200mV && < 200mV ---> RO pin has undeterminated logic state


MAX487 / MAX485 datasheet says: The receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit.


Considering the following condition:

1) Supply voltage = 5V
2) DE and RE pins at level '0' constantly (that means transmit is disabled and receive is enabled)
3) A and B pins left floating (No termination resistor and no fail-safe biasing external resistors connected). Nothing connected to both pins A and B

Questions

1) Does these RS485 tranceivers have an internal pull-up from pin A to VCC and a internal pull-down from pin B do GND? (with higher values?)

2) In this condition, what should be the voltage between A and B (5V)? (Considering a very high input impedance meter)

3) At question 2, the logic level at RO pin is '1', right?

4) Now, if I put just a termination resistor (120 Ohms) between A and B, the logic level at RO will be 'undeterminated', right? I think if the internal resistors are present, they will have a high value and the voltage between A and B will be near to 0V due to the low resistance of the termination.


I really have these doubts a long time.
Someone can help?

 

[KrisBlueNZ]

I'll try to help, but don't take this as gospel.
1. No, I don't think so. The "fail-safe" feature works by guaranteeing that if the data lines are at the same voltage, the receiver output is high. So, they have changed the behaviour whereby |VA-VB| < 200mV gives an undefined output, because the output state IS defined for at least half of that 400 mV range. Because of the internal A-B resistance, if the inputs are disconnected or the bus is undriven and |VA-VB| is zero (or near zero), the receiver output will be high. Pullup/pulldown resistors aren't needed and aren't used. This is all AFAIK!
2. Zero volts
3. Yes
4. No. If you add a termination resistor, |VA-VB| will be zero and the receiver output will be high.
I repeat, this is all AFAIK, based on my previous understanding of this feature and reading the data sheets. I haven't tested this myself.
Finally, I looked at the data sheet (19-0122 revision 9 of 2009-09) and it doesn't mention anything about this feature for any of the devices covered. It states that for all devices, |VA-VB| < 200 mV gives an undefined output state. I know Maxim offers that feature but perhaps you should check that the MAX487 (still) has it?

 

[esm.]

Here is my current schematic, attached in PDF and PNG.

I would like to design this schematic for use in RS-485 buses with a maximum of around 64 nodes.

I have some doubts about this schematic yet:

1) For R3 and R8 (fail-safe biasing), I am using 560 Ohms. Somebody know which are the most commom values used for fail-safe biasing?

2) Also for R3 and R8. I am using through hole resistors (250mW / CR25 body). Could I change them to 560 Ohms SMD 1206??? (because I am already this SMD value). That would be great!

3) Would be more suitable connect the fail-safe resistors directly to EIA485_A and EIA485_B nets, instead of connecting them to A and B nets ???

4) Maybe would be better to change the values of resistors R5 and R7 (inrush protection) ???

5) The differential voltage between A and B lines of an RS-485 bus is +- 5V, according to MAX487's datasheet. Is it recommended to place a 5V TVS diode between A and B nets? Like a SM05 TVS diode, for example? For differential protection?

Some recommendations for this schematic? Change components values?

 

[KrisBlueNZ]

Re guaranteed high receiver output on undriven RS-485 bus.
The MAX48x data sheet says only that the receiver has a fail-safe feature that guarantees a logic high receiver output when the inputs are disconnected; it doesn't say how this works.
My comments on offsetting the differential input voltage threshold don't apply to the Maxim devices at all, as far as I can tell. Sipex (now Exar) make several devices that have this feature, including the SP1486E, which is designed for Profibus and has a lot of features you might like:
5V operation
1/8th unit load (up to 256 devices on the bus)
Receiver failsafe (output high) on open, short, and undriven bus
The receivers have a fairly tightly controlled differential voltage threshold of -125 mV (nominal), rather than 0V, so you don't need bus biasing resistors to get a guaranteed receiver failsafe on a normally terminated RS-485 bus.
See http://www.exar.com/connectivity/transceiver/rs485