Rs-485 Communication - Newport New Focus Picomotor 8742 User Manual

Computer Interfacing 47
5.5

RS-485 Communication

RS-485 interface can be used to communicate with up to 31 Picomotor controllers
(8742 or 8743-CL) that are daisy-chained, using a single USB/Ethernet connection.
Once the desired controllers are daisy-chained using the RS-485 link cable, any one
of these controllers can be connected to a host PC via USB or Ethernet interface.
The controller on the network that is directly connected to a host PC is referred to as
the "master controller". This controller is responsible for receiving commands from
the host PC, and sending responses, if any, back to the host PC.
NOTE: There can be only one master controller on the RS-485 local area
network (LAN).
All the controllers connected to a master controller via RS-485 link cable are
referred to as the "slave controllers". There can be up to 30 slave controllers
connected to 1 designated master controller.
When the master controller receives a command from a host PC, it first determines if
the command is addressed to it or if the command is addressed to one of the slave
controllers. If the command is addressed to it, it processes the command and sends
any responses back to the host PC. On the other hand, if the command is addressed
to one of the slave controllers, it transmits the command on the RS-485 LAN. Since
the controller RS-485 is configured in a multidrop, parallel bus topology, all the
slave controllers receive the command transmitted by the master controller. Each
slave controller then determines if the command is addressed to it, and processes the
same if it is the intended recipient. All other slave controllers ignore the received
command. If the command is a query (i.e., requires response), the slave controller
that processed the command may transmit the response on the RS-485 LAN. Once
again, all the controllers on the RS-485 LAN, including the master controller,
receive the response transmitted by the slave controller. The master controller
determines that the information it received on the network was a response to a
command it had initially transmitted on the network, and sends this response to host
PC; all the slave controllers ignore the response they received.