is possible in cycloneII device use 2 pins INPUT for differential rs422 signal? In quartus2 the differential buffer for cyclone2 is not allowed!

You may not connect RS422 directly to an FPGA. Even if a differential buffer would exist. RS422 inputs need to have a common mode range of +/- 10V. So before interfacing to your FPGA, use a RS422 differential line receiver. Regards, Ton

hi std_logic_vector! RS422 is not RS232. It is a differential line (2 wires + ground) at level 0-3.3V. it is compatible with ecl differential levels, but cycloneII has only dedicated pin for this function. I can use the lvds standard on fpga, but it works at 2.5Volt, and i use in this bank register 3 pins at lvcmos level as output (3.3 volt). regards

Hi Thielam, I know the difference between RS232 and RS422. I also know that RS422 is not 0 to 3V3. Maybe you should read: http://www.bb-elec.com/bb-elec/literature/tech/485appnote.pdf It gives you all the information differential signaling, voltage levels, common mode, grounding, and so on. One things is for sure: You should not connect RS422 directly to your FPGA. Success, Ton

hi Ton... in my case it is a "modified" rs422. it works at 0-3.3Volt. see max13433 http://datasheets.maxim-ic.com/en/ds/max13430e-max13433e.pdf can lvds pin work at 3.3 volt?

--- Quote Start --- in my case it is a "modified" rs422. can lvds pin work at 3.3 volt? --- Quote End --- "modified" RS422, that explains it all :-) Quartus will give an error if you set Vccio to 3V3 and select LVDS as I/O standard. I don't know what happens when you set Vccio to 2V5 in Quartus, but supply your bank with 3V3. I guess there's a big chance LVDS will work, at least with your "modified" RS422. But you don't get any guarantee from my side. If you going to try it, please post the result. Good luck, Ton

ALT_INBUF_DIFF in cycloneII | Altera Community

12 Replies

Altera_Forum
Honored Contributor
16 years ago
hi std_logic_vector!
RS422 is not RS232.
It is a differential line (2 wires + ground) at level 0-3.3V.
it is compatible with ecl differential levels, but cycloneII has only dedicated pin for this function.
I can use the lvds standard on fpga, but it works at 2.5Volt, and i use in this bank register 3 pins at lvcmos level as output (3.3 volt).

regards
Altera_Forum
Honored Contributor
16 years ago
You may not connect RS422 directly to an FPGA. Even if a differential buffer would exist. RS422 inputs need to have a common mode range of +/- 10V.
So before interfacing to your FPGA, use a RS422 differential line receiver.

Regards, Ton

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ALT_INBUF_DIFF in cycloneII

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