Should I use a MAX232 in my design ?

[LBNote]

Hi all,

I have a working design using a PIC 16F84A interfacing to a PC serial port.
At the moment I'm just using a few resistors in the circuit and it's working
fine. I plan at some stage in the future to sell this product so my
questions are:

1. Should include a MAX232 in my design now for reliability or any other
reasons ?
2. What are the ramifications of not doing so ?
3. Am I stressing the micro with the current circuit I'm using ?

I plan on supporting the product so I want to make sure I'm not shooting
myself in the foot by not adding in the components now is, I guess where
I'm coming from with these questions:


Thanks

 

[Ken Taylor]

Hi all,

I have a working design using a PIC 16F84A interfacing to a PC serial port.
At the moment I'm just using a few resistors in the circuit and it's working
fine. I plan at some stage in the future to sell this product so my
questions are:

1. Should include a MAX232 in my design now for reliability or any other
reasons ?
2. What are the ramifications of not doing so ?
3. Am I stressing the micro with the current circuit I'm using ?

I plan on supporting the product so I want to make sure I'm not shooting
myself in the foot by not adding in the components now is, I guess where
I'm coming from with these questions:


Thanks

My immediate reaction is "yes, use one", but if you're only travelling short
distances and it's working, then it's probably okay. Particularly as you've
no doubt tried it out on a whole variety of different motherboards to make
sure that their different serial port implementations don't affect
things....

Cheers.

Ken

 

[LBNote]

Thanks,

It's working with a 5 metre serial cable. I have tried with a couple of
different PC and it seems to be ok.
I guess I should try some more now that you mention it, maybe a lap top as
well.

 

[Ken Taylor]

Thanks,

It's working with a 5 metre serial cable. I have tried with a couple of
different PC and it seems to be ok.
I guess I should try some more now that you mention it, maybe a lap top as
well.

To be honest I wouldn't use a crude method like that over even 5 metres.
Peter had some good suggestions in another reply. A resistive answer is
pretty evil in a commercial design really, no offence intended!


Cheers.

Ken

 

[LBNote]

Hi Ken,

None Taken,

Will go down the MAX232 Road, thanks once again for your help.

 

[John Tserkezis]

1. Should include a MAX232 in my design now for reliability or any other
reasons ?

As well as all the other responses, a TTL/RS232 interface will give some high
voltage protection. We have a recent design that blew the MAX232 every so
often, problem went away when we switched to the MAX202 (the ESD protected
version). Funny thing is, the older design never suffered any RS232 problems
other than the odd one out (rare).

2. What are the ramifications of not doing so ?

Other than reliability, compatibility. Some RS232 interfaces don't quite
accept levels as being what you *think* it should be when the voltages aren't
in spec. RS232 requires at least +/- 3v, but you've obviously found out that
0v works. This is not always the case though.

3. Am I stressing the micro with the current circuit I'm using ?

Probably not- most of the time. It's just the one situation you may come
across that blows the guts out of it where you're likely to come unstuck. Bet
it's the worst possible time too.

I plan on supporting the product so I want to make sure I'm not shooting
myself in the foot by not adding in the components now is, I guess where
I'm coming from with these questions:

Go with the MAX232 (or one of the el-cheapo pin-for-pin compatibles), let it
out into the wild, and see what happens. Go for the 202 if you start seeing
them blowing.

Remember you're going to have to invert the lines before feeding to the
TTL/RS232 driver/reciever.

 

[David Milne]

Hi all,

I have a working design using a PIC 16F84A interfacing to a PC serial port.
At the moment I'm just using a few resistors in the circuit and it's working
fine. I plan at some stage in the future to sell this product so my
questions are:

1. Should include a MAX232 in my design now for reliability or any other
reasons ?
2. What are the ramifications of not doing so ?
3. Am I stressing the micro with the current circuit I'm using ?

I plan on supporting the product so I want to make sure I'm not shooting
myself in the foot by not adding in the components now is, I guess where
I'm coming from with these questions:

Thanks

1. Use the '232, or one of it's derivatives, or competitor components. Bear in
mind that Maxim sometimes stretch the delivery times, unless you are using
10's of thousands, and have delivery guarantees in place. This is one reason
why I prefer to obtain an alternative supplier as well. However, that being said,
Maxim do have a range of the '232s that have +/-15kV ESD protection. These have
the suffix 'E' after the number. There's also a version (MAX203E) that doesn't
have any external capacitors, but has 2 drivers and 2 receivers.

</RANT ON>
I just wish they'd produce a chip that has 1 driver, 1 receiver, ESD protection,
and no external capacitors, because there are so many micros that only have
one tx and one rx pin. The standard '232 part has two drivers and two receivers and
many times, half of the chip isn't used at all. That's just being wasted,
and you pay for that part of the chip that isn't used. That pisses me off a bit.
</RANT OFF>

2. You aren't using the standard voltage levels as specified by the RS-232 standards.
This can cause errors in data transmission, because levels aren't within the standard
voltage ranges (1 = -3V to -15V, 0 = +3V to +15V), ie what is the data when the voltage
is 2 volts ??. The maximum transmission distance is only about 15 metres anyway, but
you may find that some situations work better than others. This could have something
to do with the cable capacitance, which you probably wouldn't be able to control anyway.

3. Without knowing the circuit, how can we give an assessment of the current that's
flowing through the pin?

 

[Ken Taylor]

As well as all the other responses, a TTL/RS232 interface will give some high
voltage protection. We have a recent design that blew the MAX232 every so
often, problem went away when we switched to the MAX202 (the ESD protected
version). Funny thing is, the older design never suffered any RS232 problems
other than the odd one out (rare).

The ESD issue was my main beef with long lead lengths, thanks for actually
putting it into words.
I've found earthing to the critical issue when RS232 drivers start blowing,
gotta be real careful about where you put your earth points.

Cheers.

Ken