Why the 'female' connector?

PJAllen

· The BS-2 "BoE", the "HomeWork Board", the OEM·are all outfitted with·a 9-pin female connector.·
· They are programmed, serially, via RS-232 signal levels.·
· They are, obviously, not 'communications equipment'; they are in fact computers, single-board computers: terminals (terminal equipment.)·
· In the RS-232 world there are only two categories: either it's a terminal or it's a modem.· If it's not a modem then it must be a terminal.· It doesn't have to be a full-blown computer to be a terminal.· It could be·what we used to call a·"dumb terminal."
· I suppose Parallax decided to go with a female connector, in violation of the RS-232 / EIA-232 standard (DTE/terminal/male conn. &·DCE/modem/female conn.), figuring that more people have access to a serial extension cable than a null modem cable.· I'd be surprised if there was a computer anywhere into which they could be plugged directly.
· Lots of manufacturers do the same thing, only their device presents a female connector at the end of a cable. which is wired bass-ackwards, facilitating its integration by simply plugging it into the serial port on the computer (DTE/terminal/male conn.)
· All that is designing to the dysfunctional.
· I'm working with an OEM BS2, which will be in an enclosure with the 9pin connector on a panel, that I'm re-working with a proper, male, connector, which prompts my "rant."· I'll bet that somebody already beat me to it.

Jon Williams

The reality of engineering in a *perfect* world is that we're always faced with decisions, and for some those decisions will not always be perfect for every customer.· While one can argue that the BASIC Stamp is not a piece of communications equipment, it is in fact an intelligent device (jut as a modem is an intelligent device)·that could be communicated with through a terminal program (or programming software)·running on a PC -- and that's more the norm, hence the decision to use a DB-9F on our boards.· It simplifies life for users as well, as standard serial cables are far more common than null-modem cables.

If your project is indeed communicating with a DCE device, you'd want to use a DB-9M, but perhaps you'd conenct that to the I/O pins responsible for communication -- not the programming port used by the BASIC Stamp.

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Jon Williams
Applications Engineer, Parallax

allanlane5

Well, P.J., the BOE was designed to hook up directly to a PC serial port using a DB-9M to DB-9F cable, wired straight through. Thus, the programming port IS wired "DCE", not violating anybody's standards -- except apparently yours.

To the PC, it looks EXACTLY like a modem, whose purpose is to provide communication to the BS2 programming port.

Now, RS232 is extreemely flexible, so it gives you the opportunity to add your own adapter to the DCE connection to convert it into DTE wiring. And in your application, if you wire the programming port the way you describe, you'll need a DB-9M to DB-9M 'null modem' cable to connect a PC to it. If you're mostly using the programming port to command some OTHER device, then that shouldn't be an issue. You should be aware, however, that the programming port DOES use the incoming 'TX' signal to provide voltage for its outbound 'RX' signal to the other device.

PJAllen

· Yeah, thanks, or something like it, allanlane5.··Maybe you could go back and actually read my post.

· I reckon I·divined·exactly Parallax's intent in choosing to use a serial port extension cable and wrote so in my post, to wit:·"I suppose Parallax decided to go with a female connector, in violation of the RS-232 / EIA-232 standard (DTE/terminal/male conn. &·DCE/modem/female conn.), figuring that more people have access to a serial extension cable than a null modem cable.· I'd be surprised if there was a computer anywhere into which they could be plugged directly." ( emphasis added )

· The RS-232 standard is not "flexible", as you posit.· If it was flexible then it would be the RS-232 (EIA-232)·Suggestion.· The BOE, etc. are NOT modems.· Devices can be wired up as anything one wishes with the objective being expediency and/or a certain utility, notwithstanding.· You can put a female connector on a·photometer and wire it for DCE, for all I care, but it's just masquerading as a modem.

· I·have re-worked my OEM BS2, as I described, and it·has a male connector.· I use a null modem cable (a "female-to-female", which substitutes for two modems) between it and the PC's serial port and, man, I'm doing fine on Cloud-9.

· By the way, a "male-to-male" cable is not a null modem connector --·it's a null terminal connector (assuming it's wired to connect two modems together, but in a world of "flexible standards" one can never know.)· Though, again, it's just words and people can impute whatever meanings they like, apparently.

· And finally, I sure hope that nobody at Parallax figures that my desire was to put·Parallax in a pillory.· (They're a fine bunch, the lot.)· It is, afterall,·a forum -- 1 a : the marketplace or public place of an ancient Roman city forming the center of judicial and public business b : a public meeting place for open discussion c : a medium (as a newspaper) of open discussion or expression of ideas

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=forum

allanlane5

Whatever.

It sounds like you've gotten a configuration that works for you, and that's really the important part.

Post Edited (allanlane5) : 9/26/2005 2:25:16 PM GMT

Ryan Clarke

"As you would imagine, a standard DTE device should be capable of emitting and receiving a serial data stream. As you have already seen, that includes microcontrollers and personal computers in the "could be a DTE" category. Although DCE equipment can also transmit and receive a serial data stream, the primary purpose of DCE equipment is to receive the DTE-generated bit stream over an RS232 interface and convert it to a form that's suitable for transmission over a telecommunication medium. In the case of a personal computer modem, that telecommunications medium is most likely a voice-grade telephone line.

...RS-232-C standard states that physical DTE port connectors will be male and physical DCE port connectors will be female" (Fred Eady)

Now the question: when is the Stamp a DTE vs. a DCE? That depends. Are you programming the stamp from the computer? Are you running code on the Stamp and using the computer as a terminal for debug? The answer is yes, you do both. So the stamp could be considered a DTE *OR* a DCE- so it suddenly becomes a matter of perspective.

Remember, we're talking about a standard that has had to be redefined several times because of poor definition to begin with.

Ryan

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Ryan Clarke
Parallax Tech Support

RClarke@Parallax.com

cyberbiota

Sigh...

I had hoped this thread would die quietly. The RS-232C/EIA232 standard also specifies a 25 pin D-Subminiature connector, so virtually no one meets this specification anymore. But the point of having a communication standard is to successfully get two pieces of equipment to communicate. Parallax carefully diagrams for you the electrical connection, removing any real need for an adherence to standards designed to link two black boxes. You are free to use the connection they provide or you may roll your own connection, should you so desire. There is no need to rely on meeting the standard in this case. The standard exists to facilitate connection, not to hamstring the engineers with (outmoded and largely irrelevant) details. In electronics there are many fine details that need carefull attention, but the gender of the plug on a prototyping board is not one of them.

peter

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Peter C. Charles

Director, Research and Technology
CyberBiota, Incorporated
Peter.charles@cyberbiota.com
http://www.cyberbiota.com

PJAllen

I've run into·too many·bugs and needless turmoil and wasted·a lot of time as a result of engineers, I guess,·who weren't·hamstrung,·as one calls it,·by adherence to the standard or concerned with irrelevancies like·details.

Chris Savage

Gentlemen,

·· I think it's now merely an argument of perception.· Ryan's post clearly points out that the standard allows for devices that are sometimes DCE and sometimes DTE.· The BASIC Stamp fits that category and should be generally treated as a DCE device first, since it must be programmed by a computer to become anything else.· So that is it's primary role.

·· If you're going to put the BASIC Stamp into equipment in a DTE role, then it's up to you to build the correct DB9 (Or DB25) connector into your hardware to match the new role of the BASIC Stamp.· While it's in development stage on our boards it's technically DCE.



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Chris Savage
Parallax Tech Support
csavage@parallax.com

Jon Williams

Great points, Chris, and with that let's hope this thread ends here as the tone is turning a bit uncivil and that's not what we like to see in our forums.

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Jon Williams
Applications Engineer, Parallax