Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The Serial TTL connector we deserve

kohlschuetter.github.io

Post reply on HN
Print view View original

31–40 of 80 posts

Re: The Serial TTL connector we deserve

#31

Post by exmadscientist »

eqvinox wrote: post #25
foresto wrote: post #24

Earlier quoted context omitted.

My scenario is connecting terminal emulators to getty or u-boot, which I think is a common one. A blip of line noise when plugging in would be barely considered an annoyance, easily cleared by pressing Backspace. But yes, if someone happens to be using their serial line for some kind of sensitive signaling, then I would agree that choosing a more isolated connector (or just avoiding hotplug) would make sense.

Thing is, for a serial TTL connector to become ubiquitous, it needs to cover at least something like 99% of scenarios. Or maybe 95%. 3.5mm TRS ain't that , and thus just increases general diversity in connectors.

In my experience noise on hot-plugging the serial adapter is a complete nonissue. I'm sure it matters for you, sometimes. But for this kind of embedded work, I'd say it's important far, far less than 1% of the time.

Re: The Serial TTL connector we deserve

#32

Post by exmadscientist »

Geof25 wrote: post #23
wildzzz wrote: post #13

Ideally you wouldn't need to expose a TTL serial debug port to begin with. Maybe on a prototype you would want this but I'd rather just have a single connector that can expose everything (jtag or swd). Bonus points if the interface chip is on the board so it's just a USB port

Well the first problem you will hit I'd that very likely you will need to protect or isolate those lines from ESD. This will raise price of device and it will get denied just in these grounds.

ESD diode costs are nothing compared to the connector headers or even dealing with the mechanical complications of exposing Just One More Thing. Seriously, they're cheap.

Re: The Serial TTL connector we deserve

#33

Post by exmadscientist »

> I assume that all connections use 3.3V signal levels. Should I ever come across 5V signals, I would probably use a larger Julet M8 for differentiation. OpenWrt writes that some routers already ship with incompatible 1.8V or 2.5V logic. Should the time arrive that I need to work with those, I would probably use this little fella called Micro Q ....

At that point please, please, please just switch to the 4-wire configuration exposing V_IO (aka V_target aka ...). So many problems go away when you have that available. It's nice to have at 3.3V as well, but a lot of OTS 3.3V adapters aren't configured to make use of it.

Re: The Serial TTL connector we deserve

#35

Post by theamk »

on Aliexpress, Julet M6 pigtail pair is $2.99, while USB-TTL breakout (with female USB-C connector) is $1.40 - even for nice CP2102 with unique serial number.

If you have a portable device which needs serial connector, don't bother with weird connector and bespoke adapters which will get lost. Go for USB-C directly. As an extra bonus, USB-C is significantly more electrically rugged than raw UART input.

(this does not apply if you have some sort of multi-port serial port server, but those are pretty rare, and most likely use real RS232 voltages anyway)

Re: The Serial TTL connector we deserve

#36

Post by throwaway219450 »

notthetup wrote: post #2

Nice but it’s huge! I’d prefer something smaller like these 3 pin magnetic connectors from Aliexpress. https://a.aliexpress.com/_c4CtK0gj

If you have money to burn, get some LEMO push-pull connectors. They'll run you about $80 for a pair of each gender, but when you use them, you understand why trendy audio gear and instrumentation/optical OEMs use them. Occasionally when I make a project I treat myself and use them. Cheaper options are Mini XLR and Hirose HR10. Molex make a lot of wire-wire, but they're typically not rated for more than 25-50 cycles.

Those magnet couplings are nice. EDAC make a non-Aliexpress version, about $25 a pair.

Re: The Serial TTL connector we deserve

#37

Post by throwaway219450 »

eqvinox wrote: post #11
notthetup wrote: post #2

Nice but it’s huge! I’d prefer something smaller like these 3 pin magnetic connectors from Aliexpress. https://a.aliexpress.com/_c4CtK0gj

And where do I put Vref, RTS, CTS, and plug detect? That's 7 pins ;) Less of a joke though: those aren't polarized, how do you not accidentally 180° them? Are they magnetically polarized or what? [ed.: I didn't initially see it, they're mechanically polarized, one "short end" is flat, the other rounded.]

You'll be wanting these: https://www.digikey.com/en/products/detail/edac-inc/686C0731...

And yes, they're magnetically polarized according to the datasheet. To answer the sibling comment, they're through-hole soldered but frustratingly are 2.7mm not 2.54 so you'd be squeezing a little into an existing header (ignoring clearance on the side anyway). I'm not sure why the 7-pin is obsolete, they make up to 5-way active.

Or Adafruit? https://www.adafruit.com/product/5468

Re: The Serial TTL connector we deserve

#39

Post by theamk »

Aloha wrote: post #18

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 whatev…

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)

Re: The Serial TTL connector we deserve

#40

Post by amstan »

One thing that i see often: just because both your debug cable and your target runs at the same voltage (eg: 3.3V), doesn't mean it's safe to plug them together. You could easily have a situation where your target or your debugger is off, so at that point you're powering the off thing through the uart pins (the esd diode on the pin).

The safest thing is to have a 4th vref/vccio pin, then the debugger should power its tx signal from that pin, at whatever voltage it's at. Same for the RX pin, it's not nice to have a pullup to a certain voltage when the target is off.

Post reply on HN