Source Code can be found here:
Shield is here:
I wired the D+/D- to my USB/RS485 converter. Pin 9 is attached to an LED via a resistor to ground. Ground is also attached to RS485 converter. That is all.
I copied source for your reference.
/* This example code will receive the adc ch0 value from the arduino master.
It will then use this value to adjust the brightness of the led on pin 9.
The value received from the master will be stored in address 1 in its own
address space namely holdingRegs.
In addition to this the slaves own adc ch0 value will be stored in
address 0 in its own address space holdingRegs for the master to
be read. The master will use this value to alter the brightness of its
own led connected to pin 9.
The modbus_update() method updates the holdingRegs register array and checks
The Arduino serial ring buffer is 128 bytes or 64 registers.
Most of the time you will connect the arduino to a master via serial
using a MAX485 or similar.
In a function 3 request the master will attempt to read from your
slave and since 5 bytes is already used for ID, FUNCTION, NO OF BYTES
and two BYTES CRC the master can only request 122 bytes or 61 registers.
In a function 16 request the master will attempt to write to your
slave and since a 9 bytes is already used for ID, FUNCTION, ADDRESS,
NO OF REGISTERS, NO OF BYTES and two BYTES CRC the master can only write
118 bytes or 59 registers.
Using a USB to Serial converter the maximum bytes you can send is
limited to its internal buffer which differs between manufactures.
#define LED 9
// Using the enum instruction allows for an easy method for adding and
// removing registers. Doing it this way saves you #defining the size
// of your slaves register array each time you want to add more registers
// and at a glimpse informs you of your slaves register layout.
//////////////// registers of your slave ///////////////////
// just add or remove registers and your good to go...
// The first register starts at address 0
HOLDING_REGS_SIZE // leave this one
// total number of registers for function 3 and 16 share the same register array
// i.e. the same address space
unsigned int holdingRegs[HOLDING_REGS_SIZE]; // function 3 and 16 register array
/* parameters(HardwareSerial* SerialPort,
unsigned char byteFormat,
unsigned char ID,
unsigned char transmit enable pin,
unsigned int holding registers size,
unsigned int* holding register array)
/* Valid modbus byte formats are:
SERIAL_8N2: 1 start bit, 8 data bits, 2 stop bits
SERIAL_8E1: 1 start bit, 8 data bits, 1 Even parity bit, 1 stop bit
SERIAL_8O1: 1 start bit, 8 data bits, 1 Odd parity bit, 1 stop bit
You can obviously use SERIAL_8N1 but this does not adhere to the
Modbus specifications. That said, I have tested the SERIAL_8N1 option
on various commercial masters and slaves that were suppose to adhere
to this specification and was always able to communicate... Go figure.
These byte formats are already defined in the Arduino global name space.
modbus_configure(&Serial, 9600, SERIAL_8N2, 2, 2, HOLDING_REGS_SIZE, holdingRegs);
// modbus_update() is the only method used in loop(). It returns the total error
// count since the slave started. You don't have to use it but it's useful
// for fault finding by the modbus master.
holdingRegs[ADC_VAL] = analogRead(A0); // update data to be read by the master to adjust the PWM
analogWrite(LED, holdingRegs[PWM_VAL]>>2); // constrain adc value from the arduino master to 255
The use of the enum instruction is not needed. You could set a maximum allowable
size for holdinRegs by defining HOLDING_REGS_SIZE using a constant and then access
holdingRegs by "Index" addressing.
holdingRegs = analogRead(A0);