Modbus Communications

The Modbus MotionBASIC Extension (MBX) allows ORION motion controllers to communicate using the Modbus network protocol. The Modbus factory data transfer network provides a convenient and cost-effective method for data communications between ORION motion controllers, programmable logic controllers, man-machine interfaces, host computers and other intelligent devices which support it.

Modbus Architecture
The Modbus network is a single master, multi-drop network which supports up to 247 slave devices. An ORION motion controller with a Modbus MotionBASIC Extension (MBX) supports one Modbus port as either a network master or slave depending on the software configuration selected. The preferred physical layer for the Modbus network is four- wire RS-422/485 asynchronous serial communications (ORION includes two ports). Three-wire RS-232 communications may be required in some installations. In this case, additional hardware is required. Modicon J478 modems can be used for communication over long distances or to provide multi-drop operation using RS-232 devices.

Modbus Plus
The NM-BM85-000 BridgeMux can be used as a gateway between a Modbus Plus network and up to four ORION motion controllers configured as Modbus masters or slaves.

Modbus Protocol Support
ORION motion controllers with the Modbus MotionBASIC Extension can be configured to operate as a Modbus master or as a slave in either ASCII or RTU mode. The baud rate can be set from 1200 to 38.4K baud. For a full description of the network modes and functions, refer to the Modicon publication PI-MBUS-300, Modbus Protocol Reference Guide.

Hardware Interface
ORION motion controllers can interface a Modbus network through one of its built-in RS-422/485 serial ports or via one of the optional RS-232 or RS-422 communications adapters.

Installation
To install Modbus communications in an ORION motion controller:

• connect one of the integral ORION RS-422/485 serial ports for the physical communications link,
• make sure the MotionBASIC System Card containing the Modbus MBX is installed
• configure and test the interface, and
• map register numbers to any MotionBASIC variables containing data to be communicated over the network.

Operation
Modbus is a Factory Data Communications Network and requires that you map register numbers to the MotionBASIC variables containing data to be communicated over the network. 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, other than initiating commands as the Modbus master, all Modbus communications are handled in the background by the real-time operating system, transparent to your MotionBASIC program. Heavy Modbus traffic may slow program operation however, due to the lack of a co-processor to support communications processing. Modbus is generally used to transfer integer variables and I/O information, but Modbus communications between ORION motion controllers supports the transfer of all MotionBASIC data types including integers, longs and sets as well as single and double precision floating point.

Modbus Master Support
As the Modbus master, an ORION motion controller can initiate all Modbus command functions. Frequently used functions are implemented as individual built-in MotionBASIC statements. All other functions, including those not listed in the chart, can be generated by your program using a powerful custom function statement. Data sent or received by these functions is obtained from, or loaded into, registers that are mapped to MotionBASIC program variables.

Modbus Slave Support
As a Modbus slave, an ORION motion controller can respond to the Modbus functions shown in the chart. The Read/Write Holding Registers functions can transfer up to 120 registers per transaction. The Read Coils and Read Inputs functions can transfer up to 512 coils and/or inputs per transaction.

Data is transferred to, or received from Modbus functions using register numbers mapped to MotionBASIC program variables. All Modbus slave communications occurs in the background and is transparent to your MotionBASIC program. In addition, Modbus slave communications is configurable to allow any network changes to certain registers to trigger a specified subroutine. This feature makes it easy for Modbus communications to initiate an action to be taken by any ORION motion controller.

Modbus Functions Initiated by ORION as the Master

*--Modbus master functions not listed are supported using the "Custom Command" feature

Modbus Functions Recognized by ORION as a Slave

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