Hi, I found a bug in InterconnectedSystem when connecting two systems with different dt, one of which is None. I believe the correct semantic is for the new system to take the dt of the non-None system, but the way it is implemented, the order of the systems matters.

import control as ct
sys1 = ct.tf2io(ct.tf('s'))
sys2 = ct.NonlinearIOSystem(lambda t,x,u: x, name='sys2', states=('x'), inputs=('u'))
ct.InterconnectedSystem((sys2,sys1))

This code works, but changing the order of sys1 and sys2, as in ct.InterconnectedSystem((sys1,sys2)) gives a bug:

TypeError Traceback (most recent call last)
in
2 sys1 = ct.tf2io(ct.tf('s'))
3 sys2 = ct.NonlinearIOSystem(lambda t,x,u: x, name='sys2', states=('x'), inputs=('u'))
----> 4 ct.InterconnectedSystem((sys1,sys2))

/usr/local/lib/python3.7/site-packages/control-0.8.3-py3.7.egg/control/iosys.py in init(self, syslist, connections, inplist, outlist, inputs, outputs, states, params, dt, name)
892 dt = sys.dt
893 elif dt != sys.dt:
--> 894 raise TypeError("System timebases are not compatible")
895
896 # Make sure number of inputs, outputs, states is given

TypeError: System timebases are not compatible