Can an "RS-232 Serial Ethernet Adapter" do this...?

Factual Questions
1

Can an “RS-232 Serial Ethernet Adapter” appear as a TCP device to software on a PC, such as terminal emulator software, and appear as an RS232 device to an instrument plugged into it? Maybe this sounds trivial, but the ones I read about on Amazon seem to appear to Windows as just another COM port, and I DON’T want that. I’m trying to talk to RS232 devices using a laptop without an RS232 serial port, and the several USB adapters I’ve tried so far just do not work. I suspect something about the laptop is interfering with whatever method they use to look like an RS232 port to the system. What I want to try is an adapter that does NOT look like just another COM port, to try working around whatever failed mechanism was supposed to execute that trick. In other words, I want to try running a terminal emulator (Realterm, Terra Term, PuTTY, etc) that is trying to telnet to an IP address, I don’t want to fool it into thinking there’s a fake COM7.

Thanks!

2

A lot of the devices come with Virtual COM software for convenience but can be configured to be a telnet device as well.

I believe someting like this can.

Or something liek this.
Note: I do not have experience with either of the two devices so YMMV.

3

I don’t understand our question. A serial port is a hardware device. TCP/IP is a communications protocol that can operate over a variety of hardware - even carrier pigeons. Look up the OSI 7 layer model.

If you’re trying to communicate with a RS-232 device you need a physical RS-232 port at some point in the chain. You might try getting a PCMCIA card with a proper hardware serial port.

Perhaps a silly question, but you are using the right sort of serial cable, aren’t you?

4

Quartz, I understand TCP/IP protocol and RS232 hardware ports are in different magisteria, so to speak. I know more about the layer model than I think I need to for this one.

I have tried several USB adapters and one PCMCIA card with a proper hardware serial port, with almost no success. One of the adapters works with Hyperterminal but locks when I try to turn capture-to-file on or off, and drops lots of characters. Several of the adapters worked for one afternoon with Tera Term, but then they all stopped working. All the adapters that required software installation reported that the software and hardware was working properly by its own internal testing. However, excepting these few small and partial successes, none of the combinations of hardware and terminal emulator program I have tried have worked at all.

Usually the problem is that terminal programs give “no port” errors on loading or say no ports are available after they load. Even if the program loads and says there are ports, it isn’t functional. For example, RealTerm indicates the RX line is going high and low when my instruments talk, but displays no characters, and if I try to send anything, it shows the CTS, RX and TX lines going high and staying that way. This is a known RealTerm issue “with some adapters”.

I’ve verified that there is some variety in the chipsets used in the adapters I am trying, which include 3 recommendations from our IT department and 4 adapters I bought from Amazon based on their user comments having small numbers of complaints of the product not working.

My working theory at the moment is that there is just something weird about this Dell laptop and the way it treats RS232 ports, or more accuractely what is supposed to appear as one to software on the laptop. So, I want to try hardware that appears to be something other than an RS232 port, from the point of view of software on my machine.

If this doesn’t work, I may look further into a $400 RS232 protocol analyzer I found on the web, to see if their dedicated software includes functions like moving text in and out and sending and capturing from/to files. Not that that is what they intended it to do, but pretty clearly their software would interact by different means than Hyperterminal looking for COM7.

And if that’s a bust, I wind up having to maintain another PC just to use its serial port, and all the overhead (antivirus, corporate security, fending off obsolescence) that that entails.

Cables? I have a few, including just jumpering pins 2 and 3 together for loopback, but mostly am using null modem cables that work elsewhere.

5

Have you tried putty (a freebie) for terminal emulation? It will do com ports and telnet.

I’m not aware off-hand of any software that does that IP communication with a real COM port, let alone a USB one.

6

Are you sure the problem isn’t voltage levels? RS-232 properly outputs a logical one as negative 3 volts or less (and logical zero as 3 volts or more), but many USB adapters output 0 or 5 volts since that’s what’s available from the USB line, and most equipment works fine. Laptop COM ports can have the same problem.

On your setup, you see the RX line changing (the adapter can handle negative voltage inputs), but the TX and control lines are probably getting stuck since the equipment isn’t seeing the right voltage levels.

7

Do you have a Dell docking station available? They usually have a RS-232 port on them.

8

I’ve used devices from Lantronix and Comtrol on a number of projects. They seem to work well enough.

9

I second this concern - I’ve had problems in the past talking to some RS232 devices because of voltage level concerns - I’ve made my own RS232 level translator using a simple TI voltage level translator chip and two RS232 port on it to simply act as a go between, but you might be able to buy something similar, or better yet, an RS232 -> USB translator with a proper level translator chip inbetween.

10

I found this model while searching a bit:
http://usdigital.com/products/usb-232/

It claims to output -5 v for the low voltage. I’d suspect that this is sufficient for almost all equipment since it’s within RS-232 specifications, though it wouldn’t surprise me if there were something out there that needed more, since at least for PCs, -12/+12 v is the “traditional” range.

I’ve used translator chips myself, though my experience is with Maxim. The MAX220 is cheap and easy to use–just add a few capacitors. The MAX233 is only a bit more expensive and even easier to use, since it requires no external caps. Both generate a negative voltage reference internally with only a positive supply.

11

Md2000, I am using PuTTY, per the OP.

Quartz, I have a docking station, and it has no RS232 port. IT tells me RS232 ports are getting harder and harder to find on new equipment.

Waffle Decider, thanks for the recommendations; I have both pages open now. Can you tell me, if I tell my terminal emulator to telnet to the address of these devices, will I be able to send and receive ASCII and send and capture text files, the way I used to do when I told my terminal emulator to use the COM port I used to have?

About the voltage levels - why would this make the terminal emulator app give errors on loading, or report that there are no COM ports? And wouldn’t this only limit the ability to transmit, not the ability to receive?

12

What devices are you trying to talk to? Or, more specifically, what software are you trying to use?

For the industrial devices I have to communicate with, 90% of the USB to serial devices don’t work, but in every case this Keyspan USB / Serial adapter works fine.

The Ethernet to Serial converters don’t do such a good job in most cases unless your application was designed for them. For long haul, I do use a pair (designed that way) of Ethernet to Serial, and Serial to Ethernet, but it’s a pure serial connection on each side of the link.

13

I regularly need to talk to thermometer readouts, dataloggers, PLCs, temperature controllers, stepper motor controllers, and similar devices. Right now on my desk I have a DataTaker DT800 datalogger, a handheld Garmin GPS, and an oem Garmin GPS, to use as test cases. Usually, though, the software gives errors on loading or shows no COM ports, so the test signal isn’t even in play yet.

The software I have been trying includes Hyperterminal (which is very klunky for instruments), Realterm, Tera Term, Terminal (by Bray), PuTTY, and DeTransfer and DeLogger (both of which are dedicated software for the DT800).

The USB to RS232 adapters I have tried include an EasySync ES-U-1001-R10 US232R-10 (which was bundled with the DT800), a CP TECHNOLOGIES CP-US-03 recommended by our IT, a Cables Unlimited USB-2920, a Tripp-Lite USA-19HS, a Micro Innovations (no part number and can’t find the instructions at the moment), an ATEN UC-232A, and a Manhattan 205146. The last four were highly recommended by users on Amazon and had large numbers of users and small fractions of them rating the product 1 or 2 stars. I haven’t tried the one you link.

14

Maybe this just became moot. IT just told me they’d give me a second PC with a serial port on it.

15

I deal with a lot of serial stuff. Keyspan (now Tripp Lite) makes the best adapters that work almost every time. The el-cheapo ones based on the Prolific chipset sometimes work, but most of the time do not, especially when you are trying to push a lot of data.

16

Note the Tripp Lite didn’t work either.

17

Hope your new PC is working out…

Send and receive ASCII is no problem. Sending and receiving files may be more problematic. If it uses protocols such as XMODEM or something like that, I’m not sure your typical TCP/IP based terminal emulator would support them over the socket, since those are historically UART based protocols. I’ve never had the need to do that, so I’ve never looked into that.

Of course, another assumption is that you won’t need to do anything unconventional with the handshaking lines, since you obviously can’t directly control them over a standard telnet session. Most common configurations (RTS/CTS, DTR/DSR, and a few others) are supported and can be pre-configured on the device (along with other serial parameters).

18

Waffle Decider, thanks - the only files I want to send and receive are ASCII text files. The way I understand it, “Send text file” (or whatever the program names it) is not sending files per se, but rather sending the contents of files, and is a feature entirely inside the terminal emulator program. There’s no file protocol involved. The terminal emulator program would send, byte by byte, all the characters inside the file. There would be no header information, no redundancy (other than parity bits in the RS232 signal itself), no attributes, no structure. It’s like hiring a blazing fast typist. And vice versa for receiving. Hyperterminal calls this “Capture text to file”, I think, which makes this more obvious.

Do you think I have this right, and do you think if the terminal emulator program supports both telnet to a static IP address and sending text from / capturing text to a file, the ethernet serial adapter would work?

I think I can stipulate no hardware flow control for any of my applications. IIRC I have not used it anyplace within the last 5 years or so.

Thanks!

19

Oh ok, I’ve just looked at Hyperterminal. If you’re just using its ‘Send Text File’ and ‘Capture Text’ functions, I think it should be ok. The ‘Send File’ and ‘Receive File’ operations are the ones that I think might get interesting.

You can try to look for user manuals and other documentations on their website. They’ll probably give you a better feel of what those devices do.

Since you’ve had trouble with a few other adapters, there’s no guarantee that your device will be compatible with those embedded serial to ethernet servers. All I can say is that they are much more likely to work than those cheaply designed USB dongles, but your mileage may vary, etc.

20

I suspect that there is something special about the way my laptop emulates an RS232 port using the USB, something that doesn’t work. A software setting, maybe, an otherwise little-used hardware component, a bit someplace that caught a cosmic ray.

I recently bought a fancy datalogger that hosts its own web page for configuration and for data access. I installed their software, which lets you plug the logger into a USB port and browse to it. Then I plugged it in, and it opened Internet Explorer, which told me it couldn’t display the page. It said I might not be connected to the Internet (gotta love how Microsoft takes the same approach to troubleshooting browser trouble whether you were using the web just 30 seconds ago or have never ever even had internet service).

Then I found out that, internal to the datalogger, there is a USB to serial converter, which connects to the RS232 communications system internal to the datalogger. Tthis brand of logger has always had RS232 as its default primary connection means, and I think only recently added USB by this method. Now we have a hardware box and software designed exclusively for use with one another, and my PC won’t do it, because somehow it figured out there was a USB to serial converter hiding somewhere in the system.

Waffle Decider, I think I am going to order one of your linked Ethernet devices and try it. Getting that to go might be less extra work than maintaining a separate PC. I’ll post results. Thanks for the suggestion and your further advice!