Sign in to follow this  
Followers 0
BobLfoot

1756-DNB Scanner and Bus Off Condition Recovery

6 posts in this topic

The last three times my Electrical Utility Company has been kind enough to wake my by flashing all the lights in the house one of our control systems {A 1756-L63 rev 16.7 with 1756-DNB Rev 7.2} has come up in the bus off state. The User manual says cycle power to the module and doing this recovers it. I know this is happening because the DeviceNet Power Supply is slower to recharge and Power the Network than the CPU Backplane Power Supply get's Power to the card. Can I disable the scanner cards in the Power Up Handler {ie set Local:3:O.CommandRegister.Run = 0 or Local:3:O.CommandRegister.HaltScanner = 1} and wait for the DNET Power to be present {Local:3:I.StatusRegister.DnetPowerDetect = 0} before Running the scanner {ie set Local:3:O.CommandRegister.Run = 1 or Local:3:O.CommandRegister.HaltScanner = 0}. If that won't work can I monitor the Status Register {Local:3:S.ScannerStatus} and when it = 91 {Bus Off} issue the Scanner Reset {Local:3:O.CommandRegister.Reset = 1}. My end goal is for the network scanner to come up and not go Bus Off following a Utility Loss. Any body else had to deal with this.

Share this post


Link to post
Share on other sites
From Robert D. Law's DeviceNet Bible: "DeviceNet specifications state that 24VDC power supplies must power up to full voltage in no more than 250ms under full load and 7000uf capacitance. Failure of power supply to follow this specification will cause devices to Bus Off (solid red network LED) when AC is switched on to the power supply. If devices go Bus Off when AC is applied, cycle DC power off then on at the supply and see if devices still Bus Off. If they don't then power supply is out of spec. If they still Bus Off, then you have other issues with your network." Another entry from DeviceNet Bible: Maybe you could initiate a reset to the scanner after a set time after PLC first scan.

Share this post


Link to post
Share on other sites
I found the rung with an unconditional OTE Local:3:O.CommandRegister.Run for my 1756-DNB in slot 3 and another rung with an unconditional OTE Local:4:O.CommandRegister.Run for my 1756-DNB in slot 4. I've modified these to be XIO Local:3:I.StatusRegister.DnetPowerDetect OTE Local:3:O.CommandRegister.Run and XIO Local:4:I.StatusRegister.DnetPowerDetect OTE Local:4:O.CommandRegister.Run. We'll see if waiting to start the scanner until power is present solves the problem. This is the middle of summer after all and I expect a few more power blips before cold weather.

Share this post


Link to post
Share on other sites
I usually have a delay before starting the DeviceNet Scanner. Gives time for things to power up/settle down Try: TON Timer ? ? BST XIO Timer.DN OTE Local:3:O.CommandRegister.Reset NXB XIC Timer.DN OTE Local:3:O.CommandRegister.Run BND 5 seconds in the preset should be more than enough

Share this post


Link to post
Share on other sites
I thought about that sparky. I'll try it next if the Power Detect is to fast a start.

Share this post


Link to post
Share on other sites
thats what i do is wait 10 seconds after 1st scan. even qualify the 24vdc with a seperate input to the plc to say I have 24vdc and wait for a specified time then set the run bit on the devicenet card. Obviously after power up the last thing people want to do is get everything back to running after a power hit or failure. These timeouts will have long past.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0