USB to Serial adaptor

Does anybody have a favorite USB to RS-232 adaptor that they can recommend? Evidently some are not created equal and have had some issues.

A colleague in France reported that he had found some discrepancies in how the grounding was accomplished: "It appears that the RS232 plug body is NOT always bound to USB plug body, nor to the pin 5 (ground) of the rs232 plug, so that finally there is not the same ground reference between the PC, the RS232/USB plugs and then the RS-232 peripheral. This is the origin of MANY communication errors."

In his case, the adaptor was run on a laptop that had no connection to earth. The RS-232 peripheral (which we produce) was earthed.

tia - Oppie

Reply to
Oppie
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I have a Kingwin adapter cable:

- USB shield connects to braided cable shield.

- Not sure if cable shield goes anywhere at the other end. No apparent electrical connection.

- Pin 5: < 1.3 ohms to USB ground pin (I measure ~0 ohms to computer chassis once plugged in).

Tim

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Reply to
Tim Williams

- DE-9 shield not connected.

- Voltage levels meet RS-232 requirements: symmetrical (+/-) 6-7V, with some ripple from the charge pump evident. Not good for a really long cable (where the original +/-15V would be better suited), but certainly legitimate, not some phony +3/-0V "compatible" interface.

Chip is Prolific (probably PL2303), which has TTL/LVCMOS interface. Don't know what RS-232 driver was used; probably a MAXx232 clone.

- Works well in my limited experiments; seems to have sufficient baud rate precision and selection range. Doesn't seem to transmit frames continuously, but that may be a limitation of the program's sending capability, or the drivers or USB. Does correctly recieve continuous frames (i.e., complete stop bit immediately followed by a start bit).

Tim

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Reply to
Tim Williams

We've been using these under WIN7, works on XP.

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The DB9 shell is floating. Pin 5 is connected to the USB shell. I have yet to discover a problem with them. The USB cable is 3 feet long to boot.

Cheers

Reply to
Martin Riddle

I've always liked the Keyspan adapters; the single USA-19 works well and the dual USA-28 (Macintosh style connections) has full-differential transceivers (RS-422) and works at impressively high data rates (I've tested up to 240 k baud, it claims 1 M bps).

Reply to
whit3rd

As a general issue, I prefer the ones which use the FTDI chipsets. The Linux and Windows drivers both seem to be good, and the FTDI chips include unique device IDs masked into the chips that the drivers can use to properly distinguish one

As I understand it and read the specs, all properly-defined RS-232 interfaces should be looking at the voltages between the signal lines, and the "common ground" (pin 5 on a DE-9 per TIA-574).

I don't believe it's proper for a device to depend on there being any particular connection to the DE-9 metal shell - this may be connected to the host device's chassis ground, or to the "protective ground" RS-232 signal (pin 7 on a DB-25, not defined on a DB-9), or to nothing at all. On a DB-25 serial connection, pints 1 and 7 (signal ground and protective ground) are often connected together but this isn't required.

With RS-232, you cannot depend on their being a common chassis-ground reference between the two devices at either end of the line. They may be quite some distance apart physically, may be powered from different AC mains, and there may be significant ground currents between the two. It's even possible for one or the other to be deliberately isolated from ground, for safety reasons, or to be powered like a laptop through an adapter which doesn't carry the building safety ground through to the device.

If you've got some sort of terminal device which won't work correctly with even a simple three-wire connection (TxD, RxD, and common ground), then I'd say that the device is probably at fault, and not the USB-to-serial dongle.

Reply to
David Platt

I would avoid FTDI parts like the plague. Often, they work well.

Cheers

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Syd
Reply to
Syd Rumpo

Yep. Anything with an FTDI chip. I'm forced to deal with ancient Motorola radio programming software that talks directly to the serial UART. Without a proper emulator, the Motorola software just doesn't work. I've done battle with a small assortment of USB to serial cables using Prolific, TI, and SiLabs. Each will work with some combination of computah and radio, but only the FTDI chip seems to work with all combinations.

I recently purchased several of these: Of course, the drivers on the CD were somewhat out of date, but that was easily fixed with a download from the FTDI web pile:

Note: I'm using DOSbox to slow down the computer so that the old Motorola software will run. That doesn't work with Prolific cables but works fine with FTDI.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

On a sunny day (Wed, 9 Oct 2013 16:09:06 -0400) it happened "Oppie" wrote in :

I have a bunch of Logolink adaptors, those are cheap, small, and so far have no problems.

I dunno, I killed one by runnig 20A or so from a transmitter 12V supply via the ground. Not all USB cables have screen connected either I found.

In his case, the adaptor was run on a laptop that had no connection to

Reply to
Jan Panteltje

I have had good experience with this one:

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Cheers

Klaus

PS Some USB to serial converters do not have good control of the handshake lines, so the f*'' up the timing. AFAIR the MCT behaved well

Reply to
Klaus Kragelund

Many thanks for all the replies.

My colleague shared that these seem to work in his application

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While these did not

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Still sorting out the whys and wherefores...

Reply to
Oppie

This is the only model I found that works correctly with any equipment that's not broken:

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expensive yes, but worth it.

Reply to
Cydrome Leader

USB is pretty sensitive to ground bounce. We've wiped out usb ports because of that. The shorter usb converters may work because they are less sensitive to noise due to the short cable length (one has the chip in the USB connector end, the other is 12" long). As for the 4 port powered hub, might be the power supply is robust and external.

I wonder if a usb isolator will make the longer cables work.

Cheers

Reply to
Martin Riddle

I go out of my way to use FTDI chipsets. I never had an issue with them.

Reply to
miso

Ditto this. I use FTDI with old serial radios. They work fine.

Prolific is probably second best. But I have had issue with them as OSs changed. FTDI keeps the drivers up to date for old products. That is a rarity these days.

Reply to
miso

Make sure the driver is up to date on

This is a FTDI chipset device, so it should be good.

Reply to
miso

Nor did I for years. Now I do: their support is poor and (at least some of) their parts are borderline.

Cheers

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Syd
Reply to
Syd Rumpo

Care to elaborate? I have used both Prolific chips and FTDI and never had a problem with either. What problems did you find? Which parts are "boarderline"?

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Rick
Reply to
rickman

I have to be at least a little suspicious of these sellers.

"This is an Industrial Grade Product using a Philips FTDI Chip Set."

Since when was Philips involved with FTDI??? Oh yeah, since when was $30 a "low" price for a USB serial interface? FTDI chips are used in a many, many products at a much lower price, often under $10.

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Rick
Reply to
rickman

The current head scratcher is that the Maxtrac software won't run on anything faster than a 486, yet I have a Panasonic CF-25, with a Pentium 166 that runs it just fine. No clue why (yet).

The problem with Prolific adapters are counterfeit chips. I bought about 6 different USB to serial adapters on eBay for about $2.50/ea from a mix of vendors. I was trying to find an adapter that would work reliably with DOSbox and MoSlo. The included drivers would install, but if I used the drivers from the Prolific web pile, it would refuse to recognize the adapter. That's intentional as Prolific is defending itself against counterfeits. Of course, *ALL* the adapters I bought on eBay were counterfeit, which should offer a clue as to the extent of the problem. Caveat emptor.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

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