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I am making a system which accepts signals (UART) from an external board. These signals are received by my MIcrocontrollers and further processed. Now, as part of the initial documentation I asked the client for the voltage levels of his boards as I did not know what voltage levels he would be sending (it could be coming 5V,3.3V or even very low voltage 1.8V based MCUs). As I did not know his MCUs I asked for a straightforward specifications of voltage(mainly the Voh,Vol, Vih and Vil). He sent me a document stating that my UART(in other words the UARTs in MCUs) should be TIA/EIA-422-b compliant. The screenshot is as shown below - Screenshot

Now, this is quite bizzare as TIA/EIA-422-b is a RS422 standard specification and RS422 is differential. So, I will need a UART to RS422 level shifter in between, right ?

Also, from the specs of RS422 I understand that they are +-10V signalling. Right ?

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    \$\begingroup\$ They are +/-2.5V differential. \$\endgroup\$ – Marko Buršič Sep 25 '15 at 9:07
  • \$\begingroup\$ thank you. But, is the screen shot that I shared and my ubnderstanding in sync ? I will be needing an UART to RS422 level shifter, right ? \$\endgroup\$ – Board-Man Sep 25 '15 at 9:11
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    \$\begingroup\$ Yes, you need a shifter and you must have proper transmission line. This is not always obvious. \$\endgroup\$ – Gregory Kornblum Sep 25 '15 at 9:33
  • \$\begingroup\$ @MarkoBuršič. How can you say +/-2.5. Some even have +/-2V(maximintegrated.com/en/app-notes/index.mvp/id/723) and in this link (rs485.com/rs485spec.html), the receiver is +/-10V rated !!!! Also, my UART will be only RS422 compliant in electrical specs, not in speed and being differential. \$\endgroup\$ – Board-Man Sep 25 '15 at 9:42
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    \$\begingroup\$ It can't be RS422 compliant if it's not differential, it has no sense. Newer devices have 5V supply and they povide 0 to 5V output signals biased at 2.5V, thats where +-2.5Vdiff comes from, and they can accept receive signal levels +-10V, they are rs422 compliant. Best solution is to buy a rs422/rs485 optoisolated transceiver module with dc/dc converter. Also RTS signal is ussualy needed to switch output tristate from HiZ to operation. \$\endgroup\$ – Marko Buršič Sep 25 '15 at 10:49
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Why don't you take a typical differential transceiver with the necessary protection and full compliance to the standards?

Here is the minimalistic option http://www.ti.com/product/ds8921

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