Reverse Engineering RS-485 Signals - Floating Issue

Question

I'm currently trying to reverse engineer RS-485 signals, see waveforms below.

As you can see, Signal 1 (Yellow) and Signal 3 (Purple) are a differential pair and seem to be floating.

Here's a closeup of Signal 1's wavefrom:

I'm trying to use a MAX485 Arduino module which has the following schematic:

Can anyone advise me as to whether this is possible with this module and, if so, what's needed?

If not possible, is there another IC I could use to achieve this floating signal?

I've recreated the circuit above with a MAX491EPD IC and removed any biasing/termination resistors. The issue is still there, see output below:

I'm starting to think it's an Arduino code issue. Code below:

const int EnTxPin =  2;  // HIGH:Transmitter, LOW:Receiver

void setup() { Serial.begin(19200); Serial.setTimeout(100);
pinMode(EnTxPin, OUTPUT); digitalWrite(EnTxPin, HIGH); }

void loop() {
Serial.write(0x02); Serial.write(0x50); Serial.write(0x30); Serial.write(0x32); Serial.write(0x00); Serial.write(0x56); Serial.write(0x03); Serial.write(0x30); Serial.write(0x38); delay(57); Serial.write(0x02); Serial.write(0x50); Serial.write(0x30); Serial.write(0x32); Serial.write(0x00); Serial.write(0x57); Serial.write(0x03); Serial.write(0x30); Serial.write(0x39); delay(1); delay(10000); }

Am I missing something here?

Answer 1

According to the MAX485 datasheet, page 7, the driver unit outputs are high impedance (disabled and "disconnected") from A and B when DE is low. In that condition (DE=0) what remains on nodes A and B are the resistances R5, R6 and R7, from the schematic:

^{simulate this circuit – Schematic created using CircuitLab}

In the absence of anything else driving A or B high or low, those resistors form a potential divider which sets the potentials of nodes A and B near the midpoint of the supply, close to +2.5V. This is the same behavior as what you see in your 'scope plots of the system you are trying to replicate.

As others have pointed out, I wouldn't call this state "floating", because potentials are very deliberately defined by those resistors, which provide a relatively low impedance path (equivalent to about 10kΩ) to DC. A better word is "biased".

Answer 2

Yes that module can be used. RS-485 signals float when no driver is enabled. It can be set to receive only and passively listen to the communication, if the bus is already terminated then you might want to remove the 120 ohm termination resistor and maybe the bias pull-up and pull-down resistors.

Answer 3

you should remove the termination resistors from that module all the resistor right of the max485.

other than that yes, that module will work.

spoiler alert: if I'm reading that display correctly the baud rate appears to be about 9600