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Serial is under-rated

yeek Page Icon Posted 2021-02-22 11:51 PM
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While I was waiting for a long-running Cognos report today I swung by the dead-tech room. In that room I found an old Avocent serial console server. Now I should mention a couple days back I picked up a serial cable for my HP 320LX from UsedHandHelds. You can already see where this is headed.

So of course I grabbed a terminal emulator from the SCL and plugged in the Avocent. After hooking up the cables I was standing there with my little palmtop and a full Linux terminal. Pretty cool. Only thing is the Avocent is, well enormous!

So of course now I had to order a serial GPIO hat for one of my raspberry pi’s. Turns out it’s a standard part of the raspbian OS to run a serial console, just run raspi-config and turn it on. All I gotta do now is wait for the post and I can use my HP 320LX as a screen and keyboard for my pi!

This is really appealing as I can access the pi without needing to have it and my palmtop on the LAN. No need for a PCMCIA ethernet card, no need to run a WEP WiFi access point, no freaky hard-to-find hardware or software. I don’t even have to jump through the hoops to get the CE version of OpenSSH up and running. This is freaking great!

Anyone else take advantage of this? All I could find were a few years-old threads, it’s such a great function I’d have thought it’d be more popular.
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stingraze Page Icon Posted 2021-02-23 12:25 AM
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I just thought of a similar thing too.

I was thinking of connecting my Sigmarion 1 to Raspberry Pi Zero via serial (the one that connects to PC). I haven't really tried this yet, and have little experience with serial, but I hope to share mine if it works here.
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yeek Page Icon Posted 2021-02-23 12:55 AM
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I think if you use one of those USB to serial cables it shows up as like /dev/ttyUSB0 or something. As long as your terminal emulator allows you to specify that as the connection device I would think it’d work? I wonder what kind of speed those can do, I might have to pick up one of those too.

I’m starting to wonder if HP didn’t use some sort of obscure but off the shelf connector for the serial cable. If i can find one at mouser or something I may be able to just get one of the smaller GPIO serial boards and wire that right to the connector. Slap one of those little battery packs to the pi and all I need to do is plug it in and connect.

One bummer is the serial connection seems to eat batteries, I had three bars left when I started messing with it and was down to one after all of like 15 minutes. I should probably stop buying the no-brand AA’s from the drugstore... Still serial has to be less power-hungry than a PC Card right?
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jorel101 Page Icon Posted 2021-02-23 2:05 AM
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It sounds similar to what user joval is doing with his jornada 720 using devuan to access his Pi. This video may interest you. https://invidious.kavin.rocks/watch?v=2xBK5eD1j9w
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ntware Page Icon Posted 2021-02-23 4:39 AM
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yeek - 2021-02-22 7:55 PM
Still serial has to be less power-hungry than a PC Card right?

Not necessarily. Serial runs on 12V (yes, twelve!!!) where everything else runs on 3.3V, so there is a big step up converter wasting energy somewhere. Also, when you connect plain serial to a Linux console, all you can get is a TTY. In Linux language this means the good and old terminal. That’s it. There is no way to copy files over, open multiple connections, or access X, the graphical environment.
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munchausen Page Icon Posted 2021-02-23 10:16 AM
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You can do multiple consoles over a serial connection with something like tmux or screen. But you can also do networking... you can configure PPP using pppd on the pi to run over serial to either CE or Linux on a jornada, the same as the old "direct cable connection" in windows. Then you can run telnetd (or better ParanoidTelnetD https://github.com/ColumPaget/ParanoidTelnetD) on the pi and use telnet for console work (I am assuming there is a telnet client for CE available somewhere).

The big problem is of course speed... serial (on the jornada at least) is far slower than a 10/100 card, max 230400 bps (maybe limited to less in CE). Although worth noting that the serial port connected to the IrDA transceiver can do 4mbps. I'd be interested to know what actual speeds can be achieved from a 10/100 card.

In terms of power consumption, even with a serial transceiver (which typically will output +/-8V rather than the standard +/-12V, because that's enough for pretty much any 232 device) the power consumption is going to be way lower than a PCMCIA card. I could find actual figures, but I'd imagine serial idles at almost nothing and uses max maybe 30mA for TX, whereas PCMCIA ethernet is going to be perhaps 100 mA or more at idle and several hundred mA for TX.
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yeek Page Icon Posted 2021-02-23 12:03 PM
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ntware - 2021-02-22 11:39 PM
In Linux language this means the good and old terminal. That’s it. There is no way to copy files over, open multiple connections, or access X, the graphical environment.


Who needs X11 or is it xfree86 now? I haven't been keeping up with the names. For a long time my only computer was a wicked old Toshiba that struggled with X, sure it was an early 90's laptop, one of those thick as a brick beige ones that used a regular appliance cord for the power connection 'cause the transformer was part of the body. I was using it in the early 2000's and then it was just running irssi or vi 90% of the time. I think the only window manager I even had on it was twm. All the good stuff runs on terminal anyway. And oh God screen! I used to remember all those crazy shortcut keys, split the terminal in half, toggle to the next terminal, Ctrl+a+"just about every other key"! LOL

Hearing serial is 12V is a revelation. The expansion board I got says it's doing 3.3V on it's end. I'm thinking it'd be interesting to measure what the HP 320LX is actually putting out over serial. If pin 5 is ground what do I use for hot, TXD? Or can I measure across pins 1 and 9? One of the things I was reading said supply voltage could be 5V, 9V, or 12V between 1 and 9. Makes me wish HP put out a repair manual for the 320LX, or maybe they did and I just can't find it. Guess I'll be reading up on serial connections!
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munchausen Page Icon Posted 2021-02-23 4:06 PM
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Almost all devices use a maxim chip for logic level conversion, or a clone from another company, and they tend to produce +/-8V. Although the RS-232 standard is +/-12V (or even +/-13V), pretty much any RS232 device will accept +/-8V. I can see from my pictures of the J720 motherboard that it uses a maxim MAX32something but can't make out the last digits.

If the TX line is doing nothing (idle) you should see it sitting at some positive or negative voltage between about 3V and 15V, relative to ground. On a 9 pin RS232 cable pin 5 is GND and 2 and 3 are RX/TX.

That raspberry pi hat looks like it's got a maxim clone on it, I can't make out the numbers but it's 3s and 2s! And you can see the capacitors for the charge pumps. Maxim have a mind boggling selection of serial line level converters but most of the part numbers start either max32 or max33.

By the way, don't be confused with so-called "TTL RS-232" which is not really RS232 at the physical level, it is logically RS232 but with no specific connectors and uses 0V and one of 5V, 3.3V or 1.8V for signalling, and therefore doesn't actually conform to the standard. It's what the line level converters convert from/to. But if both devices use TTL levels you don't need the converters.

Edited by munchausen 2021-02-23 4:22 PM
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yeek Page Icon Posted 2021-02-23 6:05 PM
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Yeah the actual connection is sort of confusing to me. Like one of the boards to add a serial port I saw only actually connected like 4 or 5 wires to the GPIO, and then the pinout on the port showed it was only using half the serial connector for anything, the other pins were just dead. Even the one I bought, it has those SPI0 and SPI1 sockets, and they look like they're additional serial connections (SPI apparently stands for serial peripheral interface).

Really wish I was one of those guys who could look at a circuit diagram and understand it. As it is I can look at it and copy it on a prototype board, but that's about it. Somehow or other I gotta make the leap to actually knowing the ins and outs of what I'm looking at. Maybe if I just do it long enough...
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