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Controlling a PowerFlexDrive on Modbus

RTU with Explicit MessagesOverall Description

This application note provides some guidelines for controlling PowerFlex drives on Modbus RTU using

explicit messages. This application note details a MicroLogix 1100 PLC being used as the controller, but

the guidelines can also be used with other controllers capable of Modbus RTU.

Background

As Modbus RTU is built into all PowerFlex 4 class drives, controlling a drive on Modbus RTU provides

one of the most cost effective methods of control. The MicroLogix 1100 PLC when used as a Modbus

RTU master, communicates with only one drive at a time, using explicit messages. This means that with

more drives on the network, the longer it takes to communicate with all the drives. When explicit

messages are used for control, it is important to understand how a message instruction works, and the

risks and associated limitations when used.

A message instruction is configured as part of the ladder logic in the user program. When the ladder logicis processed and the message instruction is enabled, the contents of the message instruction are passed

to the transmit buffer, or the message queue if the buffer is already being used. The message instruction

occupying the transmit buffer will then be executed when the ladder completes, within the service

communication interval. If the destination drive receives the data correctly, then it issues a reply, and the

message instruction completes and releases the buffer for further use.

To control a PowerFlex 4 class drive, the PLC needs to send a Message write instruction, for the Logic

control and the Speed Reference. To get the status of a PowerFlex 4 class drive, the PLC needs to send

a further Message read instruction, for the Logic Status and the Speed Feedback.

Although the specification of Modbus RTU allows for a maximum of 247 nodes, divided into a maximum

segment size of 32 nodes, in practice the number of drives that can be controlled is significantly less than

31, as the response time is cumalative and becomes too large. As further drives are added, each

message needs to be sequential to the last, and so the time response to the control word is increases

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proportionally to the number of drives. Therefore it is important to review any installation with regard to

response time and Health & Safety implications. The limitations are detailed below:

Limitations

Most PLCs have a real time scheduler to control the program and normal I/O, with messages

being unscheduled at the end of the ladder logic (service interval). Therefore a message may be

be updated within 1 to many program scans, which means that it is difficult to determine how long

the drive will take to start / stop when the command is given. Therefore all equipment used in thismanner, should be subject to a risk assessment, taking into account the mechanical and electrical

implementation.

Each Message instruction requires a response from the drive within the message timeout time

before it is complete. As futher drives are added, they will need additional explicit messages for

control. Therefore the messages need to be carefully sequenced / managed within the program,

so that messages that are queued up dont time out before they have a chance to be transmitted.

This will limit the number of drives that can be connected on Modbus.

Modbus RTU uses the serial physical layer RS485 which is capable of in excess of 1000 m.

However tests have shown that small drives using fast IGBTs produce significant electrical noise

and interference. This noise / interference is picked up in the RS485 cabling, and may result in

the drive dropping off the network, or indeterminate behavior. Therefore any installation shouldfollow the good practice guidelines outlined in GMC-RM001A-EN-P, routing the Modbus cable

well away from the drive and other cabling. Experience has shown that Modbus cabling should be

kept to a minimum. i.e. within a cabinet.

To check for loss of communications, a time-out value (parameter in seconds) can be setup in

the drive to issue a drive fault. This needs to be set to a value, in excess of that taken to

message the maximum number of drives, and so needs to be in the order of seconds.

Controlling drives on Modbus is better suited to installations that do not need dynamic control of

equipment. Usually starting and stopping a drive is more important than controlling the speed

reference, which may not change for hours at a time. Therefore for some critical installations,

PowerFlex drives could be started / stopped via their digital inputs, with the reference still comingvia a message over Modbus. Alternatively PowerFlex 4 class drives have the facility for a

broadcast Message over Modbus RTU, which could be setup for all the drives to be started

/stopped together, or could be setup for all the drives to be sent a common speed reference. This

cuts down on the number of Messages and the response time of each drive.

Although Modbus control of PowerFlex 4 class drives is cost effective, the items above effectively limit the

number of drives that can be controlled. Tests have shown that with the default communications, it takes

approximately 50ms to get a status update from each enabled drive. The PowerFlex40P requires 100ms

as 2 read requests are made. This means that the time to communicate with further drives is cumulative,

with 16 PowerFlex4M drives taking 800ms. Therefore it is important to review any installation with regard

to response time and Health & Safety implications.

For an alternative / more dynamic method of control, or for a larger number of drives, PowerFlex drives

can use implicit I/O control. This method of control creates a connection to the drive as part of the real

time I/O control of the PLC. This method of control is used with the ControlLogix / CompactLogix PLCs,

with EtherNet/IP providing the easiest and most integrated form of implicit I/O control for a PowerFlex

drives. Typically this could be around 30 drives in CompactLogix and 60 120 in ControlLogix..

The programming software RSLogix5000 v16.x and above for ControlLogix / CompactLogix, contains

integrated profiles for PowerFlex drives that with a few clicks of the mouse create all control tags

automatically and an implicit I/O connection at the specified Requested Packet Interval to control the

drive. This connection is monitored at both ends to ensure that the PLC and drive are communicating. A

watchdog will generate a drive fault if the drive doesnt respond in the order of 100ms. As the drives areconnected on EtherNet/IP, all their parameters are visible across web / internet, making remote

diagnostics feasible. Therefore using fieldbus in conjunction with ControlLogix / CompactLogix, provides

an easier, much more dynamic, deterministic and secure method of controlling drives.