RS232/485 impromptu survey.

Async serial is never going away. The simplicity of it vs. USB or ethernet is not even a topic worth discussing - too obvious. Many, many micros will have only async on board for the foreseeable future.

As for RS232, it has disappeared from most computers - but USB to async adapters are around. This might be a good time to ask which ones work and which ones don't. Believe it or not, there are many USB to async adapters that just don't work!

The group I work with has done about 8 controllers over the last 3 years. These all include RS422 async single and/or multi-drop. None of these controllers are sold individually - they are part of a larger machine. The protocols are all passing strings around.

These async interfaces are the primary interfaces to these controllers. I can't imagine spending valuable time futzing with USB or some such modern scheme, when there's work to be done.

When the datarate of my device is sufficiently low, I integrate a serial port, usually selectable between RS232 and RS422.

These USB drivers are a pain. Who is going to promise me that in 20 years, any USB device is still functional ? They usually rely on windows drivers and MS comes up with a new concept every couple years. With Serial, and the hardware level translator in place I can be sure that the device is interfaceable in another 20 years.

Thanks to TexasInstruments, I found an RS485 driver that does 25MBit over 160m, and another set that does 3MBit over 500m. So even with a medium datarate, my devices can have a serial connection.

Yes, a serial to USB is proposed external to the device.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Thanks for the comments, please identify the TI driver.

scott

Very popular here! We sell industrial electronic modules that often wind up connected to equipment with serial ports.

Using a laptop with a USB port might be nice for doing the setup or for loading a program, but only a small part of it's life is spent doing that. And cheap converters are available for that.

--
Gary Peek                mailto:mylastname@mycompanyname.com
Industrologic, Inc.      http://www.industrologic.com
Phone: (636) 723-4000    Fax: (636) 724-2288

Let me put it this way:

Sales started asking about USB versions of our software. We use serial cables and RS-232, but some newer laptops don't have serial ports. We increased the available ports on the software from 1-4 to 1-8 and included a 3rd-party USB converter cable in the box.

Any product we have that talks to another device uses RS-232. ANY product.

--
Magnus McElroy
Electrical Engineer (EIT)
HABIT Research
(250) 381-9425

Are these distances and speeds for point to point connections only or are multidrop connections also possible ?

Profibus-DP (which is essentially RS-485 with asynch characters) is quite picky at 12 Mbit/s at 100 m. Small series inductances are needed in the main line at each DB-9 T-connector to compensate for the branch capacitance, even if the transceiver is just a few millimeters from the connector in each node.

Paul

In addition to others' comments, you can bit-bang an RS-232 interface on any microcontroller (I couldn't imagine bit-banging USB even on a processor fast enough to do it), no serial port needed (or no EXTRA one needed if the one(s) you have are dedicated to something else). If you're doing on-board interprocessor communication, you don't even need the voltage/level translation (i.e. no MAX232 chip), though most chips have some sync ports that would do better for on-board comm. Even if it's running on a 32.768kHz clock crystal and a watch battery, you can do something like 300 baud in software (I doubt you could power a USB interface with a watch battery, and certainly not for very long). Even if the controller doesn't have a timer, you can do ugly timing loops and at least transmit easily. Receiving isn't as easy as transmitting, but with a timer interrupt you can do full duplex with no problem, up to some max baud rate depending on processor throughput and clock frequency.

The next lower-level interface to the real world would be a pushbuton swich and an LED, using the same port hardware as basic no-flow-control RS232 (one output bit, one input bit), but the data rate for this is a lot slower, and a lot of people haven't even heard of Morse code...

Bit-banged USB on an AVR:

see:

for one such device.

and have a look at the user feed back at:

and for USB to 422/485 using the same chipset:

Don...

--
Don McKenzie
E-Mail Contact Page:       http://www.e-dotcom.com/ecp.php?un=Dontronics

RS-232 to VGA. Many resolutions http://www.dontronics.com/micro-vga.html
USB to RS232 Converter that works http://www.dontronics.com/usb_232.html

These transceivers are the SN65HVD23D & SN65HVD24D, replacement compatible with the industry standard DS75176, and cost 4$ in single quantities at digikey.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

All of our designs in the last two years include RS232 and/or R2485, mostly (but not only) async. Some of them have Ethernet ports too, no USB though.

Our products have to deal with a lot of serial protocols (some of them almost older than myself) and the problem we are having these days is the lack of good, hardware-assisted serial protocol analyzers. Yes, we have software analyzers, but they need PC's with 2 serial ports (almost extinct) and they always have problems with the software drivers and Windows versions... The other day, our 1989's Thosiba PC with MS-DOS

4.01 and ISA-based LM1 anayzer died, and we are still in mourning for it.

NVE has got some isolation 485 drivers that can do 35Mbit. Not sure over what cable length. (Device is the IL485). Just add a small DC-DC Converter and one has a fully galvanically isolated RS485 interface.

Regards Anton Erasmus

Thanks for the link. The point is the distance. Apparently the transmission line requires some tweaking what the above mentioned transceivers do. If ground signals are to be expected, I'd insert a magnetic coupler too.

Have a look at the TI application notes on the subject.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

We use it for all comms to electronics from other close electronics. Then to PCs or long distances we use ethernet.

100% of my new designs have RS232, RS485, or both. USB makes sense if you are connecting to a PC, ethernet makes sense if you are connecting to a ethernet network. I'm most concerned with connecting to a radio or wireless modem, located in the same box as my product, mounted on a utility pole (with a 15 inch cable).

I'm starting to see a whole new generation of spec writters though, that are only familiar with PCs. They don't seem to understand why ethernet is not the best way to connect two battery powered devices, that are a foot apart.

-Hershel

Guess why I keep my Bondwell 286 laptop. Hint: look in back.

Thad

My reason for RS-232 is realy simple, less code overhead. Why do something with 100+ lines of code when you can do it with 10? Not to mention, at least in my case, it is cheap.

Most of my apps only need to send less then 1024 bytes.

If you look around you can still find MBs with serial ports, well at least one, gigabyte does a few.

Also PCI serial cards, all though rare can still be found, I picked up a few for NZ$1 a peice. Not to mention I have a few ISA ones stock piled.

The big pain is OEM software that will not work on anything faster then a 386!!! but using programs like atslow I can get most of them working, even on a 1.8GHz P4, running MSDOS that is ;)