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I have soldered an RS485 to TTL converter on a board and want to test it. The thing for a test, I need differential inputs as DATA+ and DATA- and I don't have a sensor which outputs differential signal at the moment.

So currently below is the diagram in my mind:

enter image description here

A μC board is supposed to receive the converted TTL signal. The μC board is powered through the USB of a laptop. The laptop uses an adaptor which is not earth grounded aka floating wrt earth.

Now to create differential inputs to the transceiver, I want to use a function generator named as Fungen in the above diagram. The function generator is mains earth grounded.

I plan to use two 100 kOhm resistors and wire the middle connection to the ground of the transceiver which is also connected to the μC ground as shown.

Is this way of creating differential inputs correct without any worry about CM voltages ect?

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2 Answers 2

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Not really. All you've done is connect one of the differential inputs to ground and you're using the other input to read a single-ended signal.

Since you have the SN75176A transceiver, just use another one to convert the test signal to proper differential form.

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  • \$\begingroup\$ the laptop is ungrounded, so that might change things. \$\endgroup\$
    – Justme
    Commented Nov 1, 2019 at 15:49
  • \$\begingroup\$ Do you mean something like this: i.sstatic.net/eSf4r.png? Thanks \$\endgroup\$
    – floppy380
    Commented Nov 1, 2019 at 15:53
  • \$\begingroup\$ Yes, exactly like that. \$\endgroup\$
    – Dave Tweed
    Commented Nov 1, 2019 at 16:06
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Just for testing, I would just connect tranceiver ground to signal generator ground as again connecting floating devices with high impedances is asking for trouble. Then put generator signal to receiver positive input. Put the 100k voltage divider between 5V and ground to get 2.5V, and connect that to receiver negative input. With signal generator, output a sine or square that is between 0V and 5V.

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  • \$\begingroup\$ Could you provide a diagram? I try to draw myself from your sentences but cannot picture the diagram for some reason. \$\endgroup\$
    – floppy380
    Commented Nov 1, 2019 at 15:58

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