Only root can bind with ports < 1024 thus use port 1502.

slapos/software/osie-coupler/buildout.hash.cfg

 [instance-profile]
 filename = instance.cfg.in
-md5sum = 5e108052b05425079e29dc7015a0f535
+md5sum = 2cf859279c110850bb32c5861669025c

slapos/software/osie-coupler/instance.cfg.in

@@ -76,7 +76,7 @@ logfile = ${directory:log}/helloweb-${:kind}.log
 # This recipe will try to "exec" the command-line after separating parameters.
 recipe = slapos.cookbook:wrapper
 command-line =
-    {{ buildout['bin-directory'] }}/pythonwitheggs {{ buildout['parts-directory'] }}/osie/eggs/osie_coupler/osie_modbus.py
+    {{ buildout['bin-directory'] }}/pythonwitheggs {{ buildout['parts-directory'] }}/osie/eggs/osie_coupler/osie_modbus.py -p 1502
 # Put this shell script in the "etc/service" directory. Each executable of this
 # repository will be started and monitored by supervisord. If a service
 # exits/crashes, it will trigger a "bang" and cause a re-run of the instance.