I am facing with a problem as below, could you all please advise me to fix it:

I use Mitsubishi GOT 1000 serial and connect with 2 Delta Servo drives ASDA-A2 400W via Modbus RTU 485.
The connection is okay. However after 4-5days, the connection time out alarm appear in GOT HMI. At this time, I cannot get data from Servo drives anymore. If I want to reconnect, I have to turn off servo drives and turn on again.

Could you please advise what i should do to correct this case?

Thank you and always with my best regards,
Thang

 

Ugh... These problems tend to get messy.

So you are getting an alarm on your HMI. Is it being driven from the alarm output DO ALRM (Bit 6) from the drive? I assume so. If that's the case, you should be getting DO: WARN (Bit 10) first.

If this is the case, you should be getting a Servo Drive Fault Code" of either 19 or 20 (or both).

Looking through the manual for the servo drive, I came across this:

The Delta servo manual calls for "A silent interval of more than 10ms" for both the start and stop interval for the RTU message. The way that they present it in the manual implies that you need this interval at both the beginning and the end of the message (so 20ms). This doesn't match what the Modbus spec calls for (See section 2.5.1.1 of the "MODBUS over Serial Line Specification and Implementation Guide" http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf).

So if your HMI is adhering to the spec, then it could cause problems on the Delta side.

The Delta does support Modbus ASCII mode, which should solve this, but the GOT does not.

However, the GOT does have a setting for a transmission delay which might do the trick, look in section 4.4.2 of the "GOT1000 Series Connection Manual" (sh080871engs.pdf) under the "Delay Time" setting. It defaults to 0ms, try changing it to 50ms (should be more than enough).

In the meantime, turning on "ASRT" (D1 signal) on the Delta when the fault occurs should clear the communications fault and get it working again without having to resort to cycling power to the drive.

Keep us posted.

 

Your intermittent problem may be due to NOT HAVING fail safe biasing on the RS485 pair. Fail safe biasing would insure the RS485 receivers are never locked up in Receive mode when the wire pair are NOT in use. See National Semiconductor AN-1057 page 10 of 10 for details.

 

Hi All,

Thank you for your reply.

The problem still occur. HMI was disconnected with Servo Drives after some days. I don't see the alarm or warning in Servo Drives so that I cannot reset/ or if i try to reset it still disconnect until i turn off the power of servo drives and turn on again. The connection automatically connect.

please give me some ideas to correct it in this case,

Thank you and best regards,
Thang

 

With other words
"After resetting the MODBUS slave you can reconnect".
Eventually the slave (Servo Drives) hangs or with other words that embedded controller software hangs.

Is there any LED or other possibility to observe if the slave software is executing it's loop? I do not think that the problem is due to problems on the level of the electrical system. Instead i think there is a problem in the firmware of the Servo Drive.

You can use a serial sniffer to watch the telegram traffic and might be able to isolate the problem. If you don't succeed you will have to ask the vendor of the Servo Drive.

 

There is no LED or anything to observe that. I also used another GOT HMI and tried to connect but I still cannot connect. Again, the reconnecting is established only when I just OFF&ON the servo drives.

Hope to hear any else advices from you all,
Thank you.

 

Next time it locks up, instead of Servo Drive OFF/ON try forcing the AB wire pair with a 1.5 V battery through a 100 Ohm Resistor.

Start by RS485 A to (+) and RS485 B to (-) and remove...if that does not restart communications then reverse the leads ie RS485 A to (-) and RS485 B to (+) and remove.

If either strategy restarts the communications then I can guarantee you it is a LACK of FAIL SAFE BIASING.

> There is no LED or anything to observe that. I also used another GOT HMI and tried to
> connect but I still cannot connect. Again, the reconnecting is established only when I
> just OFF&ON the servo drives.

 

Another possible cause is not having a 3rd wire common connection between the RS485 host and slaves.

 

Hi All,

I have tried many way but the problem is still there,

However, I would like to give you a clue that I have 2 servo drives but I only need to turn 1 of them off and turn it on. Then the connection will be reconnected again between HMI and 2 servo drives.

Could someone please give me some advices for this case and what should i do next to fix it?

Thank you and always with my best regards,
Thang

 

Hi All and Harvard,

As your advice about LACK of FAIL SAFE BIASING,

How can i do the FAIL SAFE BIASING correctly?
After testing as your comments, it shows that it may cause my problem.

Thank you and best regards,
Thang

 

See Figure 10 in National Semiconductor Application Note 1057, titled 10 Ways to Bulletproof Your RS485

http://www.avrfreaks.net/sites/default/files/RS485 Ten Ways to Bulletproof AN-1057.pdf

 

Dear All And Harvard,

Will it work if i just only do as your last 2 comments by using 1.5V battery through 100 Ohms resistor and connect it with my current RS485 system?

Next time it locks up, instead of Servo Drive OFF/ON try forcing the AB wire pair with a 1.5 V battery through a 100 Ohm Resistor.

Start by RS485 A to (+) and RS485 B to (-) and remove...if that does not restart communications then reverse the leads ie RS485 A to (-) and RS485 B to (+) and remove.

Please advise,
Thang

 

If you need more help with "Fail Safe Biasing" contact me at [email protected]