RS485 (Differential) Hookup

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NeilWicks
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Joined: Sat Aug 24, 2019 10:45 am

RS485 (Differential) Hookup

Post by NeilWicks » Sat Aug 24, 2019 10:55 am

Looking for a method to fault find Modbus RTU on RS485
Low Speed - 19.2 kbs
Looking at the 2204A
But how do I connect to TR+ and TR- (Differential)
Thanks

Gerry
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Re: RS485 (Differential) Hookup

Post by Gerry » Tue Aug 27, 2019 10:45 am

Hi Neil,

Modbus RTU can use RS485 (or now EIA485) or RS232, which use different physical connections, i.e. single ended and differential. So, the Serial Decoding provides connections for RS232, and lets you decide how to hook up the differential RS485.

So your options are:
1/ Create a Math channel of A-B, assign that to the Master or Slave and then connect Channel A to the +ve differential of the RS485 lines, and Channel B to the -ve differential.
2/ If the signal is too noisy using standard probes with a Math Channel, get a low voltage differential probe to connect to both the +ve and -ve differentials (though it may be difficult to find a low cost example).

Regards,

Gerry
Gerry
Technical Specialist

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