Dave VanHorn wrote:
Bob Ammerman wrote:
>>
>>At the receiver:
>>
>>Anything below -3v is a mark.
>>Anything over +3v is a space.
>>
>>Anything in between is 'invalid'.
>
>True enough, but look at the circuit for the 1488/89 receivers.
>They can only see positive voltage.
>

Ok, but if you use 0-5v levels, your noise margins are at best
5-0.7v = 4.3v and 0.7-0v = 0.7v. Do you really want to use 0.7v
NM over a 20-50' cable?

With +/-12v or +5/-12v levels, noise margins are many times
better.
===============


>>At the transmitter:
>>
>>Send -5v (or -6v?) to -15v (or -30v? or ??) for  a mark.
>>Send +5v (or +6v?) to +15v (or +30v? or ??) for a space.
>>
>>(does anybody know with assurance the correct range on the transmitter
>>side?)
>
>
>I don't think they spec it that way. They define a standard load, and it's
>up to you to deliver the goods.
>

I don't know the actual specs either, but seems +/-3v minimum was
the old original, and my Maxim book says MAX232 are designed to meet
EIA-232D/V.28 - which calls for +/5v minimum output levels under worst
case conditions, ie, 3K load, Vcc=4.5v, and max temperature.

Clearly, today's 115 kbps requires a little more rigor than
1970's 300 baud.

- danM

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