It should be possible to start RTDE connection with no output variables registered

[michal-milkowski]

Client library, and controller code expect at least one output variable before starting data synchronization.
This behavior is both unexpected, and difficult to handle for clients that only want to write input variables.

Proposed solution: TBD, might require code change on RTDE server side.

[robot-ranger]

we just send a watchdog recipe, but dont implement it in urscript.

i know thats not exactly the issue youre suggesting and it does consume an output, but it does allow the connection to go

[RobertAtQuestacon]

For the benefit of anyone finding this thread while having trouble starting synchronization, it is still necessary to setup at least one output for controller V5.22.0.0 before synchronization will start.
I only wanted to send one integer to the controller. The python example that google browser AI provided failed to setup an output and also used apparently obsolete RTDE commands (e.g start_file_synchronization() instead of send_start() and send_input_data() instead of send()).
I also disabled Ethernet, Profinet and MODBUS on the controller, which the browser AI and one other source advised.
This python code on a windows 10 PC now works:

import rtde
import rtde_io
import time
ROBOT_HOST = "10.0.10.82"
RTDE_PORT = 30004
con = rtde.RTDE(ROBOT_HOST, RTDE_PORT)
con.connect()
print("Connection established.")
input_registers = ["input_int_register_0"] 
inputs = con.send_input_setup(input_registers)
if not inputs:
    print("Cannot configure input registers")
    exit()    
output_registers = ["output_int_register_0"]
outputs = con.send_output_setup(output_registers)
if not outputs:
    print("Cannot configure output registers")
    exit()
print("Controller version:",con.get_controller_version())
if not con.send_start(): 
    print("Cannot start file synchronization")
    exit()

# Send data in a loop
try:
    value_to_send = 0
    while True:
        inputs.input_int_register_0 = value_to_send 
        con.send(inputs)   #previously send_input_data       
        print(f"Sent: {value_to_send}")
        value_to_send += 1
        if value_to_send > 100:
            value_to_send = 0
        time.sleep(1) 
except KeyboardInterrupt:
    pass
con.stop_file_synchronization()
con.disconnect()
print("Disconnected.")