High speed (230K) RS232 drivers?

Max 3222 ?

le

nes

Actually, worst than that. 20 pins TSSOP vs. 16 pins TSSOP Not even TSSOP 18 available.

-- Les Cargill

Yes, i know. I told me he's crazy (behind his back), but he didn't listen. Anyway, we can keep the cable as short as possible (couple inches), but the interface has to be RS232.

Did you try "But Captain, I canna change the laws a' physics!!!" ?

-- Les Cargill

le

nes

Scotty, PC claims to do 921K, we can certainly do 1/4 of that. Just do your magic.

Scotty to Spot: Can't believe everything they say, can you?

On a sunny day (Sat, 10 Dec 2011 10:33:40 -0800 (PST)) it happened linnix wrote in :

You have probably thought of that, but there exists low capacitance cable. That may make a LOT of difference.

ed

ble

one=3D

Keeping the line as short as possible will probably be the same. However, the driver chip itself needs to be fast enough.

I don't even see a data rate limit in the datasheet. The website says

You do realize unless the spec is in the electricals with limits, you can't depend on the value. Some designers print out the datasheet and black out anything without electrical limits.

FWIW, the original MAX232 is on a clunky metal gate process. The lower voltage versions were done on a faster polygate CMOS. Trouble is everyone likes the original part because it has 2nd sources.

On a sunny day (Sat, 10 Dec 2011 12:55:09 -0800 (PST)) it happened linnix wrote in :

Very long time ago the company I worked for installed a RS232 link in a factory. Several hundred meters IIRC. The guys came to me 'its not working'. It was far away, so took a day to go there with a scope, took 5 minutes to look at the waveform on the connector. Sampled it into the scope memory, showed it in the board meating (big customer). recommended lower baudrate, that was not acceptable for speed reasons, so we decided on installing an optical link.

Remember RS232 is basically a *current* drive. So if it can pull 10 mA, and you have 1000 pF cable, Q = C.U = i.t then you know the risetime (say for +- 12V swing).

t_rise = (C_cable x 12) / .01 Say maximum safe baudrate is 1 / (3 x t)

Considering UARTS normally sample in the middle of the pulse:

sample points | | ... | . . | . . | . . . | . . ...

you want at least a bit of flat tops and bottoms in the drive signal. So now you can calculate the maximum cable capacitance for a given baudrate. So it all depends on the current drive capability of that chip, not sure that is very much specified as a minimum or even maximum. You'd have to look up that spec (was discussed here before).

For a huge length of connection you should recommend an optical link. There are (or at least were) cheap RS232 to optical converters.

On a sunny day (Sat, 10 Dec 2011 23:09:31 GMT) it happened Jan Panteltje wrote in :

sorry, x 24 of course.

its late here...

Depends on your definition of cheap, but Black Box (obvious URL) still lists half a dozen RS-232/fiber converters. I bought a pair of theirs the last time I needed one, but that was probably 15 years ago.

Note that 232 isn't set up to handle ground offsets that are likely to occur over long distances. Optical is a great way to get around that problem.

Also there will be some asymmetry in rising and falling slew.

Back on the subject of high speed 232, these devices with charge pumps on-board have jitter. Charge pumps create a lot of noise, and no amount of bypass can filter the substrate noise.

I don't know if anyone specs the jitter, but it is easily measurable. There are internal limits on the jitter since ATE needs to know when to stop looking for a transition, but the limits are huge. Doing quick tests with a storage scope, you could hit the microsecond window rapidly. In communication, you are sending bits forever, so someday you will hit the perfect storm of jitter. Maybe 3uS for worst case margin. Now you can see how using a charge pump based 232 isn't all that robust for high speed 232.

So, would it help with bigger caps? Are there alternative to charge pumps?

Well, you did say the noise is internal to the chip. If that's the case, we might have to try chips w/o pumps and w external dc-dc converters.

YOu need to put a scope on the other end.

Personally, I would make them use a twisted pair converter between long runs. That seems to work very nicely.

An old product, which I have a few around here still in their boxes is called "The Black Box", they extended the range of RS232 doing balanced lines.

P.S. ALso, termination resistors are needed for long runs at higher speeds to rid the pesky little SWR's. Otherwise, those little reflections will just distort your signal and actually make it appear that your transmitter is having extra loading issues.

Jamie

We don't need a long run. It can be very short, just a few inches.

Use general purpose CMOS inverters as drivers. All modern RS-232 ICs have threshold at about +1.5V, so they work fine with CMOS levels. The only problem could be some transceivers detecting low input as "cable disconnected" condition and falling into sleep mode.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant