S908 Communications

The S908 communications adapter and S908 MotionBASIC Extension (MBX) allow ORION motion controllers to be integrated with Modicon 984 Series of programmable controllers through its Remote I/O network. This capability allows the programmable controller to be attached to an ORION motion controller through a coaxial cable and to communicate with it as a single "drop" comprised of three logical "racks" of Modicon remote I/O.

S908 Architecture

Modicon’s S908 Remote I/O network operates over coax cable at a data rate of 1.5M bits per second, and is hardened for use in factory environments. An ORION controller with an S908 Communications Adapter and a MotionBASIC MBX operates as an "I/O drop" of any Modicon 984 programmable controller. The I/O drop consists of bits of information in the shared memory interface of the S908 communications adapter which is plugged into the ORION controller. No actual I/O modules are required. The first logical “rack” in the I/O drop consists of up to 160 I/O points which are “traffic copped” to the 984 programmable controller as a collection of up to ten 16-point input and/or output modules. These inputs and outputs are then observed and controlled by the programmable controller using its standard ladder logic, and referenced by the MotionBASIC program in the ORION motion controller as additional discrete I/O points. Since all these I/O points can be updated in a single programmable controller scan, the interface is fast as well as elegant and reliable.

Adding Modbus Communications to S908

The second and third logical “racks” in the I/O drop are “traffic copped” to the 984 programmable controller as seven 64-bit output and input modules respectively. Rack 2 is used by the 984 to send Modbus query messages to the motion controller and Rack 3 is used to receive Modbus response messages. The ORION motion controller operates as a Modbus slave, with its multi-tasking operating system receiving query messages and sending responses completely independent of the user’s MotionBASIC program. The advantage of this approach is that the standard Modbus protocol can be used by the programmable controller to send and receive an unlimited number of registers as well as read and write the motion controller’s local discrete I/O. The motion controller’s analog I/O is also available to the programmable controller since all analog I/O points are referenced by the motion controller as ORMEC pre-defined variables. The relatively large size of this shared memory interface allows the programmable controller to transfer a large amount of information in a single scan (typically 4-40 msec). A maximum of 24 registers, along with 64 discrete I/O points, can be transferred in one scan. Alternatively all 160 discrete I/O points can be transferred in a single scan, while also transferring 16 registers.

Installation
To configure the I/O drop for the programmable controller, the user simply runs the standard Modicon traffic cop configuration software to configure the remote I/O drop appropriately. To install S908 communications in an ORION motion controller, simply:

• insert the S908 Communications Adapter into the ORION controller and connect the coax cable;
• make sure the MotionBASIC System Card containing the S908 MBX is installed
• configure and test the interface, and
• identify the programmable controller’s use of inputs and outputs for the virtual I/O drop, and map register numbers to any MotionBASIC variables containing data to be communicated over the network.

Operation

The S908 MBX provides software to combine the speed of a Remote I/O Network with the data transfer capability of a Factory Data Communications Network. Up to 160 points of discrete I/O for the programmable controller is “attached” to the ORION motion controller and simply referenced in MotionBASIC by using the Modicon 984 I/O point number. In addition, register numbers are mapped to any MotionBASIC variables which contain data to be communicated via Modbus messages. Both ORMEC pre-defined variables and user variables can be mapped and the mapping takes place only once in the initialization section of your program. After that, Modbus communications are handled in the background by the real-time operating system, totally transparent to your MotionBASIC program.

As a Modbus slave, an ORION motion controller can respond to the Modbus functions shown in the chart. Data is transferred to, or received from Modbus functions using register numbers mapped to MotionBASIC program variables.

All S908/Modbus communications are performed in the background by the S908 co-processor and have minimal impact on your MotionBASIC program operation. In addition, Modbus slave communications is configurable to allow any programmable controller changes to certain registers to trigger specified subroutines. This feature makes it easy for the programmable controller to initiate an action to be taken by any ORION motion controller operating as a remote I/O drop.
 

S908 Functions: ORION as Slave

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