Debug mode

Debug mode

To switch on the debug mode, where the communication details are printed:

#!/usr/bin/env python3
import minimalmodbus

instrument = minimalmodbus.Instrument('/dev/ttyUSB1', 1, debug = True)
print(instrument.read_register(289, 1))

With this you can easily see what is sent to and from your instrument, and immediately see what is wrong. This is very useful also if developing your own Modbus compatible electronic instruments.

Similar in interactive mode:

>>> instrument.read_register(4097,1)
MinimalModbus debug mode. Writing to instrument: '\n\x03\x10\x01\x00\x01\xd0q'
MinimalModbus debug mode. Response from instrument: '\n\x03\x02\x07\xd0\x1e)'

The data is stored internally in this driver as byte strings (representing byte values). For example a byte with value 18 (dec) = 12 (hex) = 00010010 (bin) is stored in a string of length one. This can be created using the function chr(18), or by simply typing the string '\x12' (which is a string of length 1). See for details on escape sequences.

For more information about hexadecimal numbers, see

Note that the letter A has the hexadecimal ASCII code 41, why the string '\x41' prints 'A'. The Latin-1 encoding is used (on most installations?), and the conversion table is found on

The byte strings can look pretty strange when printed, as values 0 to 31 (dec) are ASCII control signs (not corresponding to any letter). For example ‘vertical tab’ and ‘line feed’ are among those. To make the output easier to understand, print the representation, repr(). Use:


Registers are 16 bit wide (2 bytes), and the data is sent with the most significant byte (MSB) before the least significant byte (LSB). This is called big-endian byte order. To find the register data value, multiply the MSB by 256 (dec) and add the LSB.

Error checking is done using CRC (cyclic redundancy check), and the result is two bytes.


We use this example in debug mode. It reads one register (number 5) and interpret the data as having 1 decimal. The slave has address 1 (as set when creating the instrument instance), and we are using MODBUS function code 3 (the default value for read_register()):

>>> instrument.read_register(5,1)

This will be displayed:

MinimalModbus debug mode. Writing to instrument: '\x01\x03\x00\x05\x00\x01\x94\x0b'

In the section ‘Modbus implementation details’ above, the request message structure is described. See the table entry for function code 3.

Interpret the request message (8 bytes) as:

Displayed Hex Dec Description
\x01 01 1 Slave address (here 1)
\x03 03 3 Function code (here 3 = read registers)
\x00 00 0 Start address MSB
\x05 05 5 Start address LSB
\x00 00 0 Number of registers MSB
\x01 01 1 Number of registers LSB
\x94 94 148 CRC LSB
\x0b 0b 11 CRC MSB

So the data in the request is:

  • Start address: 0*256 + 5 = 5 (dec)
  • Number of registers: 0*256 + 1 = 1 (dec)

The response will be displayed as:

MinimalModbus debug mode. Response from instrument: '\x01\x03\x02\x00º9÷'

Interpret the response message (7 bytes) as:

Displayed Hex Dec Description
\x01 01 1 Slave address (here 1)
\x03 03 3 Function code (here 3 = read registers)
\x02 02 2 Byte count
\x00 00 0 Value MSB
º ba 186 Value LSB
9 37 57 CRC LSB
÷ f7 247 CRC MSB

Out of the response, this is the payload part: \x02\x00º (3 bytes)

So the data in the response is:

  • Byte count: 2 (dec)
  • Register value: 0*256 + 186 = 186 (dec)

We know since earlier that this instrument stores a temperature of 18.6 C as 186. We provide this information as the second argument in the function call read_register(5,1), why it automatically divides the register data by 10 and returns 18.6.

Special characters

Some ASCII control characters have representations like \n, and their meanings are described in this table:

repr() shows as Can be written as ASCII hex ASCII dec Description
\t \x09 09 9 Horizontal Tab (TAB)
\n \x0a 0a 10 Linefeed (LF)
\r \x0d 0d 13 Carriage Return (CR)

It is also possible to write for example ASCII Bell (BEL, hex = 07, dec = 7) as \a, but its repr() will still print \x07.

More about ASCII control characters is found on