I have had a couple more thoughts on your problem. Some time ago a contributor had a similar problem with I2C to RS485 converters. I had a bit of a hunt around for some info on these I had printed. I checked some TTL to RS485 units available from Core. The one mentioned in previous posts was SKU CEO5154. This has 3 resistors which everybody agreed should not be there. It has a pull up resistor to +5V from “A” and a pull down resistor to ground from “B”. I don’t know why and I think all they do is unbalance the line and lose all the advantages of RS485. The other is a permanent 120Ω terminating resistor across the line which definitely should not be there as you would be unable to daisy chain this unit.
Another 2 units CEO7681 and TEL0070 have the pull up pull down resistors but no termination.
One other circuit of unknown origin shows a dual (sda and scl) arrangement with no pull up/down resistors and a 120Ω termination inserted in or out of circuit by a 0Ω (wire link) resistor.
The 'industrial" unit you linked does not have a circuit provided but the application notes in the data sheet for the RS485 driver shows no pull up/down resistors and suggests a terminating resistor with a link to insert it across the line. I can’t find any mention of such a resistor in any text for this unit so may not be there (which is good).
What I am getting at here is that it is possible the units you are using may have these terminations installed, with or without a link and if so should be disconnected or removed. I would hesitate to leave one connected in the end unit in case they get mixed up. Leave it external so it can be readily seen.
You have not said exactly which unit you are using so I am unable to do any searching for this info.
You should be able to check if this resistor is installed internally by removing the wires and measuring the resistance across the A and B points. If there is a termination (120Ω) present it will have to be removed or any link disconnected to be able to daisy chain.
Similarly measure the resistance between A and B on your Data Collector. If the 120Ω termination is internally fitted you can leave that one there but DON’T fit another resistor.
If these terminations ARE internally fitted this would be a major part of your problem as the RS485 drivers would be seriously overloaded. I did check on that and the load resistance should be minimum 54Ω.
I still don’t know why some converters have pull up/down resistors on the A and B connections as I think everyone previously agreed that all they do is unbalance the line and lose the advantages that a balanced line presents. Unless these resistors are easily removable I personally would not use these units. But I stress that is only my preference. Perhaps someone at Core support can shed some light on this. I have not seen any explanation to date.