Chaining Maxsonar Sensors: Pulling RX pin HIGH question

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1

Hello,
I recently purchased two LV-Maxbotix EZ4 sensors maxbotix.com/uploads/MB1200- … asheet.pdf which I all ready had in possession. The sensors are used on an autonomous robot to avoid obstacles. The XL-EZ0 is used in the front of the robot while the two EZ4 sensors are used on the right and left side respectively.

I am hoping to run them sequentially to obtain range values as described by the following link from the Maxbotix website,
maxbotix.com/uploads/Chainin … oping_.pdf

I have connected the hardware as described in the schematic. The “pulling RX high” instructions however are unclear to me. According to my interpretation, I have connected the RX pin of the first/front Ultrasonic Sensor as a digital OUTPUT and turning it HIGH to pull RX pin High as you will see in my code posted below. After doing so, the front,right, left readings I get are very inconsistent. I am aware that a voltage scaling factor is required for analog readings. I will add those once i get readings that make sense.

int strobetime = 20; // in microseconds
int frontUSPin = 8; // front US pin value read through Analog 8
int rightUSPin = 9; // right US pin value read through Analog 9 
int leftUSPin = 10; // left US pin value read through Analog 10 
int frontUSswitch = 31 ;// power switch to "strobe" the RX pin of the first (front) US pin
int frontdistRead = 0;//defining variables to collect values
int rightdistRead = 0; 
int leftdistRead = 0;
int frontdistVal = 0;
int rightdistVal = 0;
int leftdistVal = 0; 

void setup() {
pinMode(frontUSswitch, OUTPUT);// define the front US pin as an OUTPUT
pinMode(frontUSPin, INPUT); // defining US signal pins as INPUT
pinMode(rightUSPin, INPUT);
pinMode(leftUSPin, INPUT);
digitalWrite(frontUSswitch, LOW); // start the program with the front Switch OFF
Serial.begin(9600); //This opens up a serial connection  
}

void loop(){
digitalWrite (frontUSswitch, HIGH); // Pull RX pin HIGH
delayMicroseconds (strobetime);// for 20uS

frontdistRead= analogRead (frontUSPin);
frontdistVal = frontdistRead; // needs Voltage scaling factor

rightdistRead = analogRead (rightUSPin);
rightdistVal = rightdistRead; // needs Voltage scaling factor

leftdistRead= analogRead (leftUSPin);
leftdistVal = leftdistRead; // needs Voltage scaling factor

Serial.print("frontdistVal:");// prints values in serial COM window
Serial.print(frontdistVal);
Serial.print(" | rightdistVal:");
Serial.print(rightdistVal);
Serial.print(" | leftdistVal:");
Serial.print(leftdistVal);
Serial.println();
delay (1000);
}

Below are the readings I get from the Serial monitor. The front,right and left obstacles are 8 inches away from the sensors. As you can see there is a spike in readings from the front sensor, which I suspect is due to incorrect “strobing”

frontdistVal:752 | rightdistVal:11 | leftdistVal:11
frontdistVal:673 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:12 | leftdistVal:11
frontdistVal:751 | rightdistVal:11 | leftdistVal:11
frontdistVal:752 | rightdistVal:11 | leftdistVal:11
frontdistVal:752 | rightdistVal:11 | leftdistVal:11
frontdistVal:25 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:11 | leftdistVal:11
frontdistVal:752 | rightdistVal:11 | leftdistVal:11
frontdistVal:752 | rightdistVal:11 | leftdistVal:11
frontdistVal:24 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:11 | leftdistVal:12
frontdistVal:752 | rightdistVal:11 | leftdistVal:11
frontdistVal:753 | rightdistVal:12 | leftdistVal:11
frontdistVal:61 | rightdistVal:11 | leftdistVal:11
frontdistVal:21 | rightdistVal:11 | leftdistVal:12
frontdistVal:20 | rightdistVal:11 | leftdistVal:11
frontdistVal:662 | rightdistVal:11 | leftdistVal:11
frontdistVal:751 | rightdistVal:11 | leftdistVal:11
frontdistVal:751 | rightdistVal:11 | leftdistVal:12
frontdistVal:22 | rightdistVal:11 | leftdistVal:12
frontdistVal:20 | rightdistVal:11 | leftdistVal:11
frontdistVal:20 | rightdistVal:11 | leftdistVal:11

Any help regarding the code would be greatly appreciated.

Sincerely,
suj10

2

The key is the text in the box at the very top of the pdf you linked to. After pulling RX high, wait 1ms (pdf says at least 20us), then change the mode of that same pin to an input. That will allow the chain to keep running properly on its own.

3

Thanks for your reply. I will give that a try. Although I am a little confused over your suggestion. You explain:
then change the mode of that same pin to an input. That will allow the chain to keep running properly on its own.

How does changing the mode to an INPUT help when I am all ready reading analog range values from the front Ultrasonic pin (connected to A8 defined as INPUT) whose RX pin is pulled high?

Thank you again.

4

As an input that pin will be in a high-impedance state which will have negligible impact on the sensor chaining circuit which operates on it own, one sensor to the next. With it left as an output and high state, the sensor that has its RX hooked to the microcontroller will be continuously enabled and thereby create cross-talk with the other sensors. As the chaining pdf says, you are just “kick-starting” the chaining operation.

5

Thanks. It makes sense now. I will give it a go and keep you updated. Thanks again.