Sending negative values via SDO

[Aragya1642]

Describe the bug
I am trying to send a negative target velocity via a SDO service call. However, when I attempt to do this, it is not possible as the data may only be sent as an unsigned integer (defined in the COWrite.srv file). The object code to which I am writing to supports negative values in the .eds file so I believe that this should work but is just not supported through the ros2_canopen package.

To Reproduce
Use the /sdo_write service to try and write a negative value to an object which should support negative values. This is the specific call I was writing which gave me an issue:
ros2 service call /motor1/sdo_write canopen_interfaces/srv/COWrite "{index: 0x60FF, subindex: 0x0, data: -100}"

Expected behavior
My motor was spinning clockwise, I would have expected the motor to spin counter-clockwise once a negative value was set. I was also expecting to receive a true status once the SDO was received which it never was and thus I never received this status.

Logs
When I had the COWrite.srv data as a uint this is the error output I see:

agoyal1642@agoyal1642-HP-Laptop-14-dq1xxx:~/Documents/Zoe2/zoe2$ ros2 service call /motor1/sdo_write canopen_interfaces/srv/COWrite "{index: 0x60FF, subindex: 0x0, data: -2000}"
requester: making request: canopen_interfaces.srv.COWrite_Request(index=24831, subindex=0, data=-2000)

OverflowError: can't convert negative value to unsigned int

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/opt/ros/iron/bin/ros2", line 33, in <module>
    sys.exit(load_entry_point('ros2cli==0.25.6', 'console_scripts', 'ros2')())
  File "/opt/ros/iron/lib/python3.10/site-packages/ros2cli/cli.py", line 91, in main
    rc = extension.main(parser=parser, args=args)
  File "/opt/ros/iron/lib/python3.10/site-packages/ros2service/command/service.py", line 41, in main
    return extension.main(args=args)
  File "/opt/ros/iron/lib/python3.10/site-packages/ros2service/verb/call.py", line 58, in main
    return requester(
  File "/opt/ros/iron/lib/python3.10/site-packages/ros2service/verb/call.py", line 102, in requester
    future = cli.call_async(request)
  File "/opt/ros/iron/lib/python3.10/site-packages/rclpy/client.py", line 119, in call_async
    sequence_number = self.__client.send_request(request)
SystemError: <built-in method send_request of PyCapsule object at 0x7b993b04f3f0> returned a result with an exception set

However, when I changed the data to an int32, this was the terminal output I saw and the motor did spin the other way like I wanted it to:

agoyal1642@agoyal1642-HP-Laptop-14-dq1xxx:~/Documents/Zoe2/zoe2$ ros2 service call /motor1/sdo_write canopen_interfaces/srv/COWrite "{index: 0x60FF, subindex: 0x0, data: -2000}"
requester: making request: canopen_interfaces.srv.COWrite_Request(index=24831, subindex=0, data=-2000)

response:
canopen_interfaces.srv.COWrite_Response(success=True)

Setup:

• Device: Linux Machine
• OS: Ubuntu 22.04.4
• ROS-Distro: Iron
• Branch/Commit: Iron but this should apply to all

Additional context
When I try and build with changing the .srv file, warnings pop up relating to this change. This will probably require further investigation as to how to approach the issue.

[danzimmerman]

If you're using the CiA402 driver and want to send signed position or velocity targets, you can use the /motor1/target service which expects a COTargetDouble with a float64 type.