Re: Hysterical question

[H.C. Croon]

> I'm trying to understand the specifications of an RS-232 receiver.
>
> Input threshold low minimum = 0.8V
> Input threshold high max = 2.4V
> Hysteresis max = 1V
>
> So, am I right in concluding that to operate reliably I need to swing
> the input over at least -0.2V to 3.4V, or do the threshold specs include
> the hysteresis?
>
>
> Peter Vogel

Hi Peter,

Read this please with a fixed font (as Courier)

Looking at the datasheet for the maxim RS232 transmitters/recievers including the MAX232A, I see:

RS-232 TRANSMITTERS:

Input Logic Threshold Low typ 1.4V max 0.8V

In my opinion that must be a mistake: the usual TTL typical threshold is 0.4V and the maximum value is 0.8V, so I read the 1.4V as 0.4V. That stems also with the term max where 0.8>0.4.

Input Logic Threshold High All devices except MAX220 min 2V max 1.4V Also this seemes to be a mistake. 1.4V must be 2.4V. This is the standard maximum high threshold for TTL. 2V is the standard typical threshold for TTL, so that number is correct.

These two signals are input signals for the transmitter, so this refers to the standard logic side.

RS-232 RECEIVERS:

RS-232 Input Hysteresis All except MAX243, VCC = 5V, no hysteresis in shdn. 0.2 0.5 1

The reciever can be switched to shut down mode, in which case there is no hysteresis.

In normal operation the input hysteresis for the reciever is typical 0.5V, but garanteed 0.2V Max is 1V. Input for the receiver is line side. Where the garanteed switching levels are + and - 3V, the maximum of 1V will allways be passed.

It seems to be not clear to everybody what hysteresis realy is.
It handles about a circuit with two states, so a Smitt trigger is an example. Being in state 1 it changes to state 2 as soon as the input voltage will be greater than threshold 1 (level switching). Being in state 2 it changes back to state 1 as soon as the input voltage is lower than threshold 2 = (threshold 1 - hysteresis).

out- |
put | -----------<-------------------state2
vol- | | |
tage | | |
| | |
| | |
|------------------->---------- state 1
___________________________________________________
th2 th1 input voltage -->
<------hysteresis------>

Circuits with hysteresis are used to garantee that after a change of state the circuit will not allmost immediately change back caused by noise, ripple or contact bouncing.

For a receiver it is a must to have hysteresis, because otherwise you will see a lot of spurous signals without any significance, leading to timing and frame errors.

BTW, the URL of maxim is
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1798/ln/en

Regards, Harry
-- Author: H.C. Croon INET: [EMAIL PROTECTED]
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