Topic: Is there something wrong with the request signal?

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November 25, 2017 at 07:54 #13051

Revolu
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I’m having some troubles trying to query a modbus slave with an Arduino through RS485.

I’ve already succeeded in querying a software modbus slave running on my PC through the USB/COM port using the ModbusMaster libray, hence it shouldn’t be a software issue.

I read about TTL and level conversions and I put on a circuit like this on a breadboard: (see the attachment: board circuit)

Using the same firmware/sketch that worked for the software slave, I connected the arduino pin TX and RX to the max485 and A and B to the modbus slave and I issued several requests.

I can see the signals converted by the MAX485 (CPA1114) though the oscilloscope and it seems to be right. The led on the modbus slave lights on as it sees a modbus transaction. Still, what I read as result of the request is always 0xE0 (invalid slave id) or 0xE2 (timeout).

I queried the slave with the same equal request using another tool (a RS485/USB converter and CAS Modbus Scanner), and it gives the expected results, that is data 0x01.

This is the code I’m running on an Arduino Ethernet (with a display for debug purpose):
<pre class=”lang-c prettyprint prettyprinted”>#include <ModbusMaster.h> #include <LiquidCrystal.h> LiquidCrystal lcd(12, 11, 4, 5, 6, 7);
ModbusMaster node(1); void setup() { pinMode(3, OUTPUT); node.begin(19200); lcd.begin(16, 2); } void loop() { uint16_t m_startAddress=1; uint8_t m_length=1; uint8_t result; digitalWrite(3, HIGH); // TX result = node.readHoldingRegisters(m_startAddress, m_length); lcd.clear(); if (result == node.ku8MBSuccess) { lcd.print("DATA:"); digitalWrite(3, LOW); // RX for (uint8_t j = 0; j < m_length; j++) lcd.print( node.getResponseBuffer(j), HEX ); } else { lcd.print("ERR "); lcd.print(result, HEX); } delay(500);
} 
These are the request signals emitted by the Arduino, that always fail to get a data response, and the other tool, that always succeed:

(Attachment: Arduino request signal)

(Attachment: USB/RS485 converter signal)

(Attachment:Overlap of the two signals)

Is there something wrong with the request signal? Am I making any mistakes in the circuit or the code?

Any pointers would be greatly appreciated.

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November 25, 2017 at 07:56 #13056

Revolu
Participant

Sorry, the code is here:

<pre class=”lang-c prettyprint prettyprinted”>#include <LiquidCrystal.h> LiquidCrystal lcd(12, 11, 4, 5, 6, 7);
ModbusMaster node(1); void setup() { pinMode(3, OUTPUT); node.begin(19200); lcd.begin(16, 2); } void loop() { uint16_t m_startAddress=1; uint8_t m_length=1; uint8_t result; digitalWrite(3, HIGH); // TX result = node.readHoldingRegisters(m_startAddress, m_length); lcd.clear(); if (result == node.ku8MBSuccess) { lcd.print("DATA:"); digitalWrite(3, LOW); // RX for (uint8_t j = 0; j < m_length; j++) lcd.print( node.getResponseBuffer(j), HEX ); } else { lcd.print("ERR "); lcd.print(result, HEX); } delay(500);
}
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