Delta VFD on Modbus / RS485

Hello all,

I am using a Delta inverter, VFD40WL21A on Modbus with a PLC. We are using the minimum communication parameters like RUN/ STOP, Direction and Speed.

I havent yet received the Keypad from the supporting concern and I used a Delta MMI to communicate to the drive to set the Acc / Decc speed. The issue is I am unable to find the parameter for Action if communication fails. It is not mentioned anywhere in the technical writeup. But being a CE and UL approved component, I am very sure the same will be there. We cannot spare an output in PLC to detect communication failure and Put OFF the drive.

The issue arises only when a communication failure occurs when the drive is running.

Kindly comment...

Regards,

Rakesh.N.

Dear Rakesh?

What model of the Delta HMI are you using?

Please elaborate on your question "The issue is I am unable to find the parameter for Action if communication fails"

what communication mode are you using?

danieluy

reference used is 5011604304 20030522 Delta User Manual VFD-L @
http://www.taeus.com/upload_images/reports/attach_161_0.pdf

The Delta VFD device is a Modbus ASCII slave.

Modbus is a master-slave protocol. Slaves do not speak unless spoken to. A Modbus slave sits and waits for a Modbus master to address it and send it message telling the slave what to do.

The Modbus standard does not address periodic or scheduled messaging, where a slave device would expect to receive a message within a certain time period. Modbus masters, on the other hand, tend to be acutely aware of timeouts involved in not receiving a reply to a message they've initiated and sent out. Modbus masters will typically (not always) provide a failure message if a timeout occurs.

In essence then, the Modbus Protocol standard does not have the means to let a Modbus slave determine that there's been a communication failure, that function would have to reside in the Modbus master, or have been addressed specifically by the manufacturer. Any manufacturer can implement Modbus as they see fit, and if the manufacturer of a Modbus slave wants to provide a mode of "fail to a given state if Modbus message is not received in X amount of time" then that's up to the manufacturer. But doing so is not an inherent Modbus slave function and is not an expected function when using Modbus.

Note, too, that the list of the Delta's recognized faults does not include communications failure, because Modbus slave mode does not really support such a determination.

Note the section on Exception Responses (pg 12)

"Exception Responses:
Except for broadcast messages, the AC drive is been (sic) expected to return a normal response after receiving command messages from the master device.

The following depicts the conditions that no normal response is replied to the master device.

The AC drive does not receive the messages due to a communication error; thus, the AC drive has no response. The master device will eventually process a timeout condition."

I believe this is the condition that you have to deal with, the failure to get a response within a timeout interval indicating failed communications. Your Modbus master will have to do the "comm failure" detection and then initiate action.

Dan

danw,

thank you for the clarification... now i understand the question..

ok...

I've been using Delta HMI for several years now and with Rakveen's question.. i believe the Delta HMI has this "Comm Timeout feature"

once the HMI (Master) didn't receive any response from the Device. after sending a Command string.

the it will have a red pop-up of Timeout error...