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Client

Pymodbus offers both a synchronous client and a asynchronous client. Both clients offer simple calls for each type of request, as well as a unified response, removing a lot of the complexities in the modbus protocol.

In addition to the "pure" client, pymodbus offers a set of utilities converting to/from registers to/from "normal" python values.

The client is NOT thread safe, meaning the application must ensure that calls are serialized. This is only a problem for synchronous applications that use multiple threads or for asynchronous applications that use multiple asyncio.create_task.

It is allowed to have multiple client objects that e.g. each communicate with a TCP based device.

Client performance

There are currently a big performance gab between the 2 clients (try it on your computer examples/client_performance.py). This is due to a rather old implementation of the synchronous client, we are currently working to update the client code. Our aim is to achieve a similar data rate with both clients and at least double the data rate while keeping the stability. Table below is a test with 1000 calls each reading 10 registers.

client asynchronous synchronous
total time 0,33 sec 114,10 sec
ms/call 0,33 ms 114,10 ms
ms/register 0,03 ms 11,41 ms
calls/sec 3.030 8
registers/sec 30.300 87

Client protocols/framers

Pymodbus offers clients with transport different protocols and different framers

protocol ASCII RTU RTU_OVER_TCP Socket TLS
Serial (RS-485) Yes Yes No No No
TCP Yes No Yes Yes No
TLS No No No No Yes
UDP Yes No Yes Yes No

Serial (RS-485)

Pymodbus do not connect to the device (server) but connects to a comm port or usb port on the local computer.

RS-485 is a half duplex protocol, meaning the servers do nothing until the client sends a request then the server being addressed responds. The client controls the traffic and as a consequence one RS-485 line can only have 1 client but upto 254 servers (physical devices).

RS-485 is a simple 2 wire cabling with a pullup resistor. It is important to note that many USB converters do not have a builtin resistor, this must be added manually. When experiencing many faulty packets and retries this is often the problem.

TCP

Pymodbus connects directly to the device using a standard socket and have a one-to-one connection with the device. In case of multiple TCP devices the application must instantiate multiple client objects one for each connection.

Tip

a TCP device often represent multiple physical devices (e.g Ethernet-RS485 converter), each of these devices can be addressed normally

TLS

A variant of TCP that uses encryption and certificates. TLS is mostly used when the devices are connected to the internet.

UDP

A broadcast variant of TCP. UDP allows addressing of many devices with a single request, however there are no control that a device have received the packet.

Client usage

Using pymodbus client to set/get information from a device (server) is done in a few simple steps, like the following synchronous example:

from pymodbus.client import ModbusTcpClient

client = ModbusTcpClient('MyDevice.lan')   # Create client object
client.connect()                           # connect to device, reconnect automatically
client.write_coil(1, True, slave=1)        # set information in device
result = client.read_coils(2, 3, slave=1)  # get information from device
print(result.bits[0])                      # use information
client.close()                             # Disconnect device

and a asynchronous example:

from pymodbus.client import ModbusAsyncTcpClient

client = ModbusAsyncTcpClient('MyDevice.lan')    # Create client object
await client.connect()                           # connect to device, reconnect automatically
await client.write_coil(1, True, slave=1)        # set information in device
result = await client.read_coils(2, 3, slave=1)  # get information from device
print(result.bits[0])                            # use information
client.close()                                   # Disconnect device

The line client = ModbusAsyncTcpClient('MyDevice.lan') only creates the object it does not activate anything.

The line await client.connect() connects to the device (or comm port), if this cannot connect successfully within the timeout it throws an exception. If connected successfully reconnecting later is handled automatically

The line await client.write_coil(1, True, slave=1) is an example of a write request, set address 1 to True on device 1 (slave).

The line result = await client.read_coils(2, 3, slave=1) is an example of a read request, get the value of address 2, 3 and 4 (count = 3) from device 1 (slave).

The last line client.close() closes the connection and render the object inactive.

Large parts of the implementation are shared between the different classes, to ensure high stability and efficient maintenance.

The synchronous clients are not thread safe nor is a single client intended to be used from multiple threads. Due to the nature of the modbus protocol, it makes little sense to have client calls split over different threads, however the application can do it with proper locking implemented.

The asynchronous client only runs in the thread where the asyncio loop is created, it does not provide mechanisms to prevent (semi)parallel calls, that must be prevented at application level.

Client device addressing

With TCP, TLS and UDP, the tcp/ip address of the physical device is defined when creating the object. The logical devices represented by the device is addressed with the slave= parameter.

With Serial, the comm port is defined when creating the object. The physical devices are addressed with the slave= parameter.

slave=0 is used as broadcast in order to address all devices. However experience shows that modern devices do not allow broadcast, mostly because it is inheriently dangerous. With slave=0 the application can get upto 254 responses on a single request!

The simple request calls (mixin) do NOT support broadcast, if an application wants to use broadcast it must call client.execute and deal with the responses.

Client response handling

All simple request calls (mixin) return a unified result independent whether it´s a read, write or diagnostic call.

The application should evaluate the result generically:

try:
    rr = await client.read_coils(1, 1, slave=1)
except ModbusException as exc:
    _logger.error(f"ERROR: exception in pymodbus {exc}")
    raise exc
if rr.isError():
    _logger.error("ERROR: pymodbus returned an error!")
    raise ModbusException(txt)

except ModbusException as exc: happens generally when pymodbus experiences an internal error. There are a few situation where a unexpected response from a device can cause an exception.

rr.isError() is set whenever the device reports a problem.

And in case of read retrieve the data depending on type of request

  • rr.bits is set for coils / input_register requests
  • rr.registers is set for other requests

Client interface classes

There are a client class for each type of communication and for asynchronous/synchronous

Serial AsyncModbusSerialClient ModbusSerialClient
TCP AsyncModbusTcpClient ModbusTcpClient
TLS AsyncModbusTlsClient ModbusTlsClient
UDP AsyncModbusUdpClient ModbusUdpClient

Client serial

pymodbus.client.AsyncModbusSerialClient

pymodbus.client.ModbusSerialClient

Client TCP

pymodbus.client.AsyncModbusTcpClient

pymodbus.client.ModbusTcpClient

Client TLS

pymodbus.client.AsyncModbusTlsClient

pymodbus.client.ModbusTlsClient

Client UDP

pymodbus.client.AsyncModbusUdpClient

pymodbus.client.ModbusUdpClient

Modbus calls

Pymodbus makes all standard modbus requests/responses available as simple calls.

Using Modbus<transport>Client.register() custom messagees can be added to pymodbus, and handled automatically.

pymodbus.client.mixin.ModbusClientMixin