My rs485 board signals seem to be clamped?

[lean]

Hi,I make a RS485 board with maxim MAX13487E chip,and it works,but seem something wrong.
The A,B line signals seems to be clamped,the A signal is clamped to 3v,it should be pull up to Vcc,and the B signal is clamped to 2.5v,it should be pull down to GND.
I check the circuit ,and change the pull up /pull down resistor to 1k and 680Ω,but nothing change,I check some other 485 module,the output signal is perfect good.
below is some pictures:

This is my board signal pic,as you can see,the channel 1 is A(yellow wave),channel 2 is B(blue wave),M is A-B, the osclliscope is set DC coupling.
But A can't reach Vcc(5v), B can't reach GND. And M voltage is no more than 0v.I think this is not the right wave.

The below two pics is some other modules' waves.
I think this is the right waves.
Purchased 485 module 1:

Purchased 485 module 2:

this is my 485 board circuit:

And I plug this board in this circuit with raspberrrypi pico board:

And there is no problem that the pico board communicate with my 485 board.

But I still think my 485 board has some flaw, so hope some help here.
Thank you!

[DrDiettrich]

@lean

The A,B line signals seems to be clamped

Don't worry about absolute levels, it's sufficient if the difference between both lines changes between positive and negative (differential signal).

[kmin]

If your differential voltage on tx is above 1.5V, your device is ok. It would probably work even much lower though.

[lean]

No,there is some case,this voltage signal will be unusable.
The reason is the pull up and pull down resistors,I find there is about 2.3v on it.
I change and measure the resistors voltage ,as follow table:
when I set the two resistor up to 1MΩ,the resistors voltage still above 2v.
when I set the two resistor down to 100Ω,the resistors voltage down to 1.5v，and then the M value will be above 0V.but still can't reach the power rail.

[jim-p]

The MAX13487 is an odd bird, doesn't operate like other RS485 ICs but you are right about it being pulled high and low.

[kmin]

On old evaluation kit maxim uses 220ohm pull-up/downs.

ps. you should use voltage translator on your pico rx

[lean]

I tried 220Ω too,and the A-B voltage change a little,the A-B value will above zero about 1v,when use 100Ω resistors,the A-B value looks good,but the ampere is too large about 15mA，the below pic is use 100Ω resistors:

And right,the raspberry pi pico board voltage is 3.3v ,lower than the 5v 485 board,I miss this when developed.
thanks

[jim-p]

@lean

raspberry pi pico board voltage is 3.3v

but that does not explain what you are seeing.

[lean]

I think this point is need to test,because in the chip datasheet,the DI input is used to drive the bus HIGH or LOW,but now the DI input voltage is not right, so the bus is not be drived HIGH or LOW as my pics.

[jim-p]

It TTL level (2V/0.8V) input so it should work with 3.3V CMOS output of the Pi

Is Vcc 5V as you show in you diargam?

[lean]

Yes,my Vcc is 5VDC,and this voltage power the Pico board too.

[jim-p]

At some point that IC is supposed to release the bus so that the voltages are set by the pull-up resistors.
I have to say that I've tried to use that IC on a bus with other normal RS485 transceivers and never got it to be 100% right.

[lean]

Sorry,I made a mistake here.
There is a 120R resistor between A and B on the board actually，but I delete it in schematic drawing.
After remove this resistor,the signal can be pulled up and down throughly.
As the below picture:

[jim-p]

@lean

After remove this resistor,the signal can be pulled up and down throughly.

but you need the 120 ohm resistor.

[JohnLincoln]

@jim-p

but you need the 120 ohm resistor.

The resistor is a terminating resistor, used to prevent reflections from the far end of the cable.

Without it the signal would become corrupted at high baud rates, and long cable lengths.

[camsysca]

Aye, but you also only need two, regardless of the number of nodes.

How many RS485 novices have tripped over the built-in 120 resistor on the MAX485 modules, when they installed their network and found signal degradation/collapse when they added the 4th, or 5th, or 6th, such module in parallel?

[lean]

@jim-p

but you need the 120 ohm resistor.

Yes,this terminal resistor may good for signal for some long wire application,I don't know,but there are 4.7k pull-up and pull-down resistors,so it consist a voltage divider,and so the A-B voltage will be small,unless use small value resistors,the voltage difference will be large.
I don't know how to select the pull-up and pull-down resistor for many nodes,I just test this module separately.

[jim-p]

Will all nodes on the nus use max13487?
What is the max number of nodes?
How long is the bus?

[lean]

this is my test circuit,no other nodes.

[jim-p]

Can you answer my questions?