# RS422 charachteristic impedance and its variants

This is regarding the RS422 bus hardware. I have a receiver that has Vih of 1.8V and a max of Vcc(3.3V).

Supposing I link a driver and receiver up. The driver,as per the EIA RS422 standards state that it can supply +/-6V differential. But since its dc levels I am reckoning it will be in the range of 6V and 0V.

Now, the receiver has a Vih of min 1.8 to 3.3V. I have posted this before but somehow I am not able to understand how I can directly interface them without a voltage divider. But using a voltage divider has its issues of impedance loading. I understand RS422 has a charachteristic impedance of 100Ohms. Now, I am just analysing the effect of passives on the charachteristic impedance on the addition of extra passives. So, supposing, I add some resistors as shown, what will the impact be ?

Also, I have a requirement that states that Differential input voltage Vdi - 600mV < Vdi < 6V(each voltage inside this range is interpreted as a signal). But, I selected a receiver as stated -

.

Will this work out ?

I just looked at this RS422 receiver - RS422 receiver. The Vid = -12V to +12V. What is the reason for such different Vids in the same standard ?

• This is a good case for providing links to the receiver and transmitter and telling us explicitly what supply rails are used for both. Also, the characteristic impedance of a cable remains unchanged whatever load you apply. – Andy aka Oct 30 '15 at 9:10
• I am studying the HS9-26CLV31RH driver and the HS9-26CLV32RH receiver IC. The supply rails are 3.3V. – Board-Man Oct 30 '15 at 9:29
• @Andyaka. So what you say is that with the circuit I showed in my question, the impedance of the line remains 120R(standard RS422) despite adding the 53R and 120R. If so, then why do we have termination resistors in differential lines if there is no impact on the charachteristic impedance ? – Board-Man Oct 30 '15 at 9:32
• The driver cannot supply 6V as an output - the power rail is only 3V3 - forget about what the standard says and concentrate what the specs of the devices tell you. – Andy aka Oct 30 '15 at 9:41
• Ok. thank you. Now, supposing the receiver has the minimal requirements of the Vid as stated before - Vdi - 600mV < Vdi < 6V(each voltage inside this range is interpreted as a signal). Now, suppose the driver is interfaced. I must be looking at the Difference in differential output (|VT| - |VT|) rating to be inside the rated voltage input(differential) of the receiver, right ? Supposeing the receiver has a Vid of 600mV to 6V (as I mentioned in my post), how can I interface them ? – Board-Man Oct 30 '15 at 9:51

It's telling you that a receive differential input lower than 100 mVp-p is going to work - it also tells you what hysteresis levels the receiver uses to reduce the effects of noise. However, this is page 1 and it's probably better to use the guaranteed numbers. If you look on page 4 it tells you what the receiver guaranteed input low level limits are ($V_{IT+}$ and $V_{IT-}$) are +/- 0.2 volts. You are needing to ensure that the peak-to-peak input voltage is greater than 200 mV. That page also re-affirms that the hysteresis is 50 mVp-p.