Hi, i have a hardware question regarding the use of the RS485 shield ( https://core-electronics.com.au/rs485-shield-11693.html ). I have connected an Arduino nano every to the RS485 shield 5v to 5v, gnd to gnd, rx to rx and tx to tx. The Shield connector is then connected for mod bus communicatio using a 2 wire shielded twisted cable. the cable connects to a db9 connector which plugs into a HMI.
Now i was reading that i may need a 120ohm termination resistor at both ends of the cable for both the rx and dx. To my understanding the RX and Tx terminals of the RS485 shield do not have a termination resistor (someone welcome to confirm).
Given that knowledge i should install a 4 120ohm resistors (1 at each end) at the end of the mobdbus cable?
Also any other hardware issues i may have overlooked are welcome.
Cheers.
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Hi Matt.
Yes the RS485 should be terminated with 120Ω a each end but where do the 4 resistors come from??
The RS485 signal is marked A and B and is a balanced pair. There is no “TX” and “RX” as such. The system TX and RX occur on the same pair but only one direction at a time, ie; half duplex.
The 120Ω termination may be on board already but when looking for a circuit or some more info I found the “Wiki” to be empty. Not a great help. You should get clarification by measuring the resistance from “A” to “B”. If this is 120Ω the termination is on board. If not it will require a resistor across this point.
The RS485 is only a transport medium using a balanced pair with all the advantages of a balanced line. Quoted maximum distance is 1200 metres. My idea of use is you provide the converter with TTL or similar, this converts to RS485 and sends down the cable where it is turned back into TTL or whatever in its original form. You need to be able to process RS485 or provide a converter at the far end.
Cheers Bob
Edit. While we are at it you may as well get something else right.
The correct description of this connector is DE9. The letter after the D denotes the shell size. DB is 25 pin size. Don’t worry too much, almost everyone including Core do this. Just thought I would throw this in to create some confusion. Using the “B” all the time creates some confusion especially when it comes to 15 pins. There would be 2. The high density version of the 9 pin connector has 15 pins, BUT there is already another 15 pin connector in the “D” series. The correct description for this connector is DA15.
DE9, DA15, DB25, DC37, DD50.
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Hi Matt
I just found a schematic for this shield.
The shield HAS A 120Ω TERMINATING RESISTOR FITTED. With this fitted as it stands this shield cannot be daisy chained. The resistor would have to be removed in all but the last unit on the chain.
There are also 2 other resistors fitted. One from “A” to 5V and one from “B” to ground. All I could see these doing would be to unbalanced the balanced line and remove the advantages that a balanced line gives. Also it would make daisy chaining these shields a bit dodgy as you would be connecting several resistors in parallel and doing all sorts of unwanted things to the chip operation. None of the application notes for this chip call for these resistors.
This has come up previously and I suggested Core support may have some sort of explanation for these components but no reply has been forthcoming yet.
Personally If I were using this shield I would be removing all 3 of these resistors and fitting the termination external where it is obvious.
Cheers Bob
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