Look at the 485 protocol it will show it's needed. Plus as above you link negs on cameras. Trust me 485 does not need a common to work. Other stuff does but not the data

Firmware is irrelevant

Don't get me wrong its good advice to prevent switching issues, but isn't needed for just data

I wouldn't as a rule common negs unless switching outputs across expanders. Electrical isolation is preferable imo

but thats the whole point of rios lims zems etc,might as well wire everything back to the panel if your worried about phase issues

And then you data line out is taken from the ethernet module ?

 

Must be, as it doesnt work on remote buildings unless common the negs, may be as they are not on the same supply.

Good point 

No the data out isnt taken from the ethernet module.

I have two data lines connected directly to the control panel and i have the ethernet module plugged onto the 4 pins.

Do you think this could cause the problem?

Well if you remember I had this issue with a 520 , a line dropping some modules , and only thing I found was the neg problem since then been a while the problem hasn't surfaced

Well if you remember I had this issue with a 520 , a line dropping some modules , and only thing I found was the neg problem since then been a while the problem hasn't surfaced

ah well the one 520 you couldnt fix and dissed a neg didnt fix the cause did it,so was it a phase problem or a supply problem or something else

Don't know geezer but problem hasn't surfaced since fixing the cable (New cable)

I didn't diss the neg? I put it back on

Good point 

No the data out isnt taken from the ethernet module.

I have two data lines connected directly to the control panel and i have the ethernet module plugged onto the 4 pins.

Do you think this could cause the problem?

 

No the length is too short and the bus too resilient for this to be an issue. But if Ethernet COM4 is dropping out too, then it rules any open on the bus out.

 

It would appear that something is dragging the bus down. A slave device can do this if it sits in write mode when it shouldn't.  An intermittent short to the A/B lines would also do the same.

Thanks but how can i tell when a slave device is in write mode?

It would need to be a faulty device to be staying in write mode.  Depending on the Ethernet firmware, the level can dip down momentarily (buggy Honeywell firmware), so you can probably just ignore on the COM4 device and the virtual keypad for it KP15.

 

I think it's probably easier for you to swap devices until you have them all done. If this was at a customer site, I would say to just swap them all and then test them back in the office. It may be better doing the same at home in order to keep false alarms at a minimum.

 

Apart from that, you'd need to connect a bus analyser/recorder and watch the traffic in order to debug the issue.

Keypad.....

Always unsets clear? Checked detail log? When they go missing maybe common link somewhere

drop the screen for a few weeks....