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RS-485 bias and termination resistors

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DSGarcia

New Member
I have a SBC that has a RS-485 port. It has jumpers for bias and termination resistors.

I would like to connect one or two led character display units generally no more than 50 feet away. If using one display unit, I will simply connect the two devices point to point. If the installation calls for two display units, I would simply like to put a 'T' connector anywhere from 1 to 10 feet away from the first display to branch to the second. The display devices will be sealed and so I do not want to have to set any jumpers or throw switches and I only want one version of the display.

How should I terminate the display devices so they will work in either configuration? (This is not a one-time deal--there will be multiple installations of the system.)

Thanks,
Dale
 

blueroomelectronics

Well-Known Member
RS485 is terminated only at the endpoints of the wire. At 50' and slower data rates 19600 you can often ignore the termination resistors.
 

DSGarcia

New Member
Bill,
Thanks. I forgot to mention that because this will be a small character display, I can use a low BAUD rate, perhaps 9600. I guess I will use gain and termination resistors on the SBC side and nothing on the display side(s).
Thanks,
Dale
 

DSGarcia

New Member
I already have some older display units built for RS-232. Since they are read-only (I only need to receive ASCII data at 9600 BAUD), do you suppose that I can simply feed in one of the RS-485 lines (-7/+12V) from my SBC--I can figure out whether A or B by trial and elimination--into the RS-232 RX receive lines (+/-15V)? for a quick and dirty conversion? The old unit uses the RS-232 serial input on a BS2. This is just to use up a few remaining units from the old stock.
Thanks,
Dale
 
Last edited:

blueroomelectronics

Well-Known Member
On some verisons of the old Cricket schematic I used to include a pair of 1k resistors as a simple converter. It was for single ended RS232 to RS485 communications but didn't support any termination.
You could try a 1k resistor between RX & B-
 
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