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0 pointsAloha9h ago0 comments

I would tell anyone who is doing a new traditional serial connector/cable to add the following -

1. Automatic DCE-DTE detection and an interface which will rewire itself as needed to be the correct way, or you automatically know DCE vs DTE by connector gender.

2. Automatic Voltage Detection - 232 levels, TTL 5v, TTL 3v - and interfaces that are isolated enough to deal with the wrong voltage (clamping diodes or whatever), or different cable sizes for each.

3. Automatic type detection - TTL/RS-232, RS-422, RS-485, or different connector types by each.

Ideally I'd do this on a 8p8c or 10p10c connector, because of ease of making cables, with various resistance values across pins 1-8, or 1-10 to tell you what kind of interface it was.

0 comments

eqvinox8h ago

At that point it's not a debug connector anymore. Note there's a pseudo-standard for V.24/RS232 on RJ45 already, and nobody uses it for debug connectors since (a) you'd need a RS232 transceiver and (b) RJ45 connectors are honking huge.

The point is to shave off the last cent, which is why you get a possibly-unpopulated 1×4 or 1×3 2.54mm header. Bonus points if the manufacturer designed series resistors into the board (let's say 0402 or even 0201) and left those out too to save the last 0.01 cent.

pocksuppet4h ago

I think they mean the debug adapter should auto-detect all these things, not the debug port. You'd have one adapter you could plug in to anything and it would usually magically work.

AlohaOP1h ago

yes, largely - even if its just pin headers adding a detect line which shows a resistance between it and ground would be an improvement.

In my working life I've seen -

at least 3 different ways to do RS-232 on 8p8c

DTE-DCE is always an issue on standard 232, otherwise I wouldn't have so many null modem cables

232 and 3.3v TTL on the same board or assembly

3.3v and 5v TTL on the same board or assembly

Inconsistent labelling.

I'm in my mid 40's and I think there is a reasonable chance something with RS-232 serial timings will outlast me, it'd be nice to make it more foolproof, as its one of a very few interfaces that will work without drivers.

I do think as a matter of standard good design practice we should be putting clamping diodes on debug ports to prevent blowing things up if hit with the wrong voltage,

AlohaOP8h ago

I'm open to other options, the biggest things is DTE-DCE auto detection

eqvinox6h ago

> I'm open to other options,

Err… if you're trying to approach the problem, the onus is on you to come up with options?

I'm not going to preclude you from being creative and coming up with a zero-cost solution that somehow magically does all this, but personally speaking I'd say the chances are minute and it's not worth my time.

I'm also not gonna try to stop you from building that thing you originally suggested, with the 8P8C or 10P10C connector and strapping pins and autodetection and whatnot, I'm just pointing out to you the issues I think it will run into. You're free to ignore my concerns. I'm a random commenter on the internet.

P.S.: calling it DTE-DCE auto detection signals to me that you haven't done a lot of work in this field; those terms are not in common use for TTL serial. People in the field would probably call it something like "TX/RX swap". (Hard to say, since it doesn't really exist.) Maybe you're an engineer for field buses or something like that though…

theamk7h ago

That sounds completely useless. Who needs a standard that is not actually standardizing anything?

If you are willing to design a board with with 8p8c connector, identification resistor, and protection parts, then you might as well put RS-232/RS-422 transceiver. Those are very cheap nowadays, might even be cheaper than the connector itself. And you standardize on crossover cables, there will be no need DCE-DTE detection.

(Not that anyone would need this today. If you are connecting to existing serial port, there is USB, and USB-serial adapters are very cheap. And if you are designing a serial-based communication system, say for the robot, then CAN or RS485 are much better choices)

mrheosuper3h ago

you are over-engineering it. At the end of the day, it's a debug connector, when you use it, you should know what you are doing. The more thing you add to debug connector, the more thing you have to debug when the debug connector not working.

Geof258h ago

What you are describing is going to be nightmare to work with - i.e. when you will have automatic detection of levels and it will decide to push RS232 into 3V3 MCU then you will have dead, maybe one of the kind prototype or dead expensive production device

AlohaOP8h ago

Depends on what you make the default as.

j / k navigate · click thread line to collapse

0 comments

eqvinox8h ago

pocksuppet4h ago

I think they mean the debug adapter should auto-detect all these things, not the debug port. You'd have one adapter you could plug in to anything and it would usually magically work.

AlohaOP1h ago

yes, largely - even if its just pin headers adding a detect line which shows a resistance between it and ground would be an improvement.

In my working life I've seen -

at least 3 different ways to do RS-232 on 8p8c

DTE-DCE is always an issue on standard 232, otherwise I wouldn't have so many null modem cables

232 and 3.3v TTL on the same board or assembly

3.3v and 5v TTL on the same board or assembly

Inconsistent labelling.

I do think as a matter of standard good design practice we should be putting clamping diodes on debug ports to prevent blowing things up if hit with the wrong voltage,

AlohaOP8h ago

I'm open to other options, the biggest things is DTE-DCE auto detection

eqvinox6h ago

> I'm open to other options,

Err… if you're trying to approach the problem, the onus is on you to come up with options?

theamk7h ago

That sounds completely useless. Who needs a standard that is not actually standardizing anything?

mrheosuper3h ago

Geof258h ago

AlohaOP8h ago

Depends on what you make the default as.

j / k navigate · click thread line to collapse