How to call the RJ45 as a serial port for interfacing?

Question

Hi,

I'm currently working on a serial interfacing project based upon connecting to the rs232 port. However, seeing as a decent number of laptops and/or PC's seem like they might have their rs232 ports disappear within the next 5 years, I was considering using the RJ45 modular port for my project (EIA/TIA 568). However, I'm not quite sure how to denominate it when accessing the port through the Win32API (I'm programming in C++). It's currently programmed to "COM1". Does anyone know how I can find out what this port is called or how to configure it so it can be used in this manner?

Thank you,

Nick Sullivan

Answer 1

I'm really confused... RJ45 ports on laptops/PCs are for Ethernet connections, aren't they? I don't understand how you could utilize one as a COM port.

If you need a serial port, and don't need ultra-low latency*, pick up a USB <-> RS232 converter, they run about $20-$30 and work just fine: from the PC software's point of view it looks like a regular old COM port.

*RS232 itself has low latency, but if you use USB as part of the serial connection, you're limited by the timing of its frames and how the USB serial converter uses them... I seem to remember USB has 1msec and 16msec frame timing; USB 2.0 appears to have 125usec microframes but I don't know if any of the serial converters manage to use them. You'd run into latency issues if you're using a command-response protocol in RS232 with lots of back-and-forth traffic.

Answer 2

The RJ45 port you are referring to is an Ethernet port, not a serial port.

If you are looking for a more current technology, you should investigate the USB interface. If you really want Ethernet, you might be interested in the XPort from Lantronix.

-- Edit --

What type of device are you looking to use as a host?

You're worried about an aging but common standard (RS-232) going away, but you're going to use a non-standard, that never was?

The documents you refer to appear to be an alternate wiring method for a typical RS-232 port.

By the way what you're referring to as RJ-45 is a wiring standard not an interface. RS-232 can be connected via DB-9, DB-25, RJ-45 or screw terminals. The voltages and signaling are part of the protocol. The connector type is determined more by convenience.

Most computers use an RJ-45 for ethernet. The only time I've seen RJ-45 connections for serial connections was on networking equipment or terminal servers.

Answer 3

The USB port has been starting to take over the duties of RS-232 for the last few years.

You should seriously consider USB as an interface for your project.

Oh, yes; that might seem like a ludicrously obvious statement -- "duh! nobody's been using Serial for a decade now".

Not so easy: there is a lot more than mice and printers out there.

RS-232 has been the preferred interface for custom-build devices, scientific instruments, and low-production devices far long after everybody started using USB for mice and high volume consumer devices.

My most direct experience comes from amateur astronomy and accessibility computer accessories for the visually-impaired. Until not too long ago, all of the above were still mainly RS232 devices, and a common headache has been finding a way to plug those in a modern laptop. RS232-to-USB consumer adapted will sometimes work, sometimes not. At least one manufacturer of accessibility devices (a braille embosser) has stated to us that they don't recommend RS232-to-USB adapters because they have had (unspecified) problems with them.

I don't know the cause. Maybe USB components are more expensive, maybe it's the need to interface with legacy devices that would be expensive to redesign; maybe it's what the engineers know. Maybe it's just "ain't broken, don't fix it" or simple inertia.

It's only been in the last couple of years, but I've finally seen a number of these devices offered with USB ports instead of serial RS232; in some cases, RS-232 versions have been discontinued. It's just taking a little longer.

Answer 4

Correct me if I'm wrong, but does the Yost wiring standard not allow one to connect to an RJ45 port in order to do serial communications?

http://yost.com/computers/RJ45-serial/

http://www.lammertbies.nl/comm/cable/yost-serial-rj45.html

All I'm wondering is how to actually access the port from the win32 API. Am I terribly off base here?

– Nicholas Sullivan

Answer 5

On the meta level, what you want to do is a bad idea. You're inventing your own standard for hardware, in a manner which is wholly inconsistent with established protocols.

You are much better directly using either USB or a DB-9 connector with RS-232. RS-232 is really quite good, but it's getting outdated(unfortunately, it's much simpler than usb). For salable products, you'd want to have both. For researchy/geek products, USB is much better.

You should be able to enumerate your com ports from Windows and open a given com port as a file(I've done it).

Answer 6

Maybe you are thinking about something like this...

http://www.comtrol.com/products/specifications/30130-1

Julianno Jungle

Answer 7

http://site.gridconnect.com/docs/PDF/xportmanual.pdf you mite get some ideas from reading this manual.

the way this manufacturer communicates with the product is by the rs-232 port on the camera and the Ethernet connection. the plug combines both the software to connect and the plug with samples.

Although I would like to have this setup , I am trying to make it myself rather than buying it. You may just consider buying the product and just shairing back to this thread the way they comuncate and the wireing of the plug or at least the fact that the conversion from serial to lan a valid solution.

I am also with a laptop that I want to talk to a CNC machine by way of rs-232 (25 Pin serial) and the LAN connection on the laptop.

It is not about keeping the past alive or saying whitch standard is better or worse than another.

It's about being able to run the CNC machine with the laptop and How to wire the RS-232 to the LAN 45rj - nine pin connector on the laptop and what software to use to make it happen.

::::::NOTE:::::: Also I wish to have the ability to use a Microsoft sidewinder joy stick that uses another connection ; a """" A Game Port """"" another failed attempt to make things easier. Run the CNC for use as a CMM or Computer Measuring Machine.

A little known fact is that people should only give information , if they know what they are talking about.