RS232 (serial) ports- DB25 vs DB9

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virge
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RS232 (serial) ports- DB25 vs DB9

#1 Post by virge » Thu Jan 06, 2011 4:47 pm

First off, I do not know hardly anything about serial ports except what I've been reading online today...

I'm trying to set up a backup computer system to interface with some industrial equipment. The current PC is utilizing 9-pin and a 25-pin serial ports. Most industrial computers (and home PCs) seem to only come with the 9-pin connections. I do see 9-pin to 25-pin converters, but it seems to me that there must be a good reason for all those extra pins. Do these converters work?
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Re: RS232 (serial) ports- DB25 vs DB9

#2 Post by RealBlackStuff » Thu Jan 06, 2011 5:27 pm

Can't tell you the reason for 25 pins (probably to do with the availability [or lack thereof] when they first came out, of certain electronic components like trigger-diodes and whatnot), but I do know that they work flawlessly.
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Re: RS232 (serial) ports- DB25 vs DB9

#3 Post by mediasponge » Thu Jan 06, 2011 5:56 pm

Ypu have ITT Cannon to thank for this mess. :roll: See: http://en.wikipedia.org/wiki/D-subminiature and http://en.wikipedia.org/wiki/RS-232. The 25 pin connectors are a legacy of Teletype terminals. Early PCs had 25 pin RS232 connectors for modems and 9 pin RS232 connectors for mice. 25 pin parallel printer ports are the opposite gender from the serial ports. Just to make things even more confusing, there were also DB25 SCSI connectors, DA15 game/Midi ports and DE9 CGA ports on real dinosaurs.
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Re: RS232 (serial) ports- DB25 vs DB9

#4 Post by virge » Thu Jan 06, 2011 6:15 pm

I've spent a few hours reading and still not quite sure what applications are limited to the DB25 adapter (vs the DB9). This line from wikipedia is worrisome: "RS-232 devices originally used the DB25 25-pin D-sub, but for many applications the less common signals were omitted, allowing a DE9 9-pin D-sub to be used." Thank you Cannon for this mess!

I assume the adapters just ignore the "less common signals." Hopefully the machinery I'm working with doesn't use those pins. If that doesn't work, I'll try a PCI serial port card. I have no idea how well a PCI card will work with a program that runs within a DOS window in Windows 98.

FYI, the manufacturer is asking for $2k+ for one of these PCs (P3, 256MB RAM, 40GB HDD, Win98 Preload). No accessories. From what I can see, the internals seem like off-the-shelf components stuffed into a rackmount case. Software fits on two floppy disks.
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Re: RS232 (serial) ports- DB25 vs DB9

#5 Post by automobus » Thu Jan 06, 2011 8:02 pm

You say the computer has both types of ports, but what about the equipment? If the equipment only uses nine pins, then you are set. There is no circuitry within the plug adapters; the nine pins pass right through. If you read the full Wikipedia article, then you will understand that so many of the pins are not needed for simple tasks. I have used 3-pin RS-232 in two applications: TI graphing calculators and an old digital camera. Both used audio style plugs.
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Re: RS232 (serial) ports- DB25 vs DB9

#6 Post by virge » Thu Jan 06, 2011 8:21 pm

automobus wrote:You say the computer has both types of ports, but what about the equipment? If the equipment only uses nine pins, then you are set.
I really don't know anything about the production equipment that the computer is hooked up too. I looked in the cabinet, and all I can say is that it's full of circuit boards. :lol: The person whose job it is to fix the machinery isn't really knowledgeable about the electronics. He's great with the mechanical portions though.

If I follow the cable to it's endpoint, I should be able to see how many wires are being used (hopefully). I guess I can get an answer if there are 9 or fewer wires, and I can compare the contacts with the diagram for the DB25 and see if it matches up to the DB9 contacts. That would give me an answer as to whether a DB9 would suffice, right?
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Re: RS232 (serial) ports- DB25 vs DB9

#7 Post by bill bolton » Thu Jan 06, 2011 8:31 pm

virge wrote: I do see 9-pin to 25-pin converters, but it seems to me that there must be a good reason for all those extra pins.
The RS232 interface set was developed in when communications interfaces had a lot more variety in hardware control functions than they do today (or have had for ~40 years) and synchronous serial wire line communications channels were still very common. The DB25 connector was needed to support all the possible signal functions, but even in my working lifetime I've rarely seen more than the basic set actually in use.

Only handful of the original set of signal lines do anything useful for run-of-the-mill asynchronous serial communications purposes that remain in regular use, and that set can be implemented within a DB9 connector... so simple DB9 to DB25 adapters do work fine for all but the most arcane purposes.
virge wrote:Thank you Cannon for this mess
Cannon had nothing to do with it... it was IBM that popularised the use of the already existing DB9 connector for RS232C, with the advent of the IBM PC.

Unless the process control kit you want to interface to is extremely old, you shouldn't have a problem.

Cheers,

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Re: RS232 (serial) ports- DB25 vs DB9

#8 Post by mediasponge » Fri Jan 07, 2011 5:40 pm

The connectors were patented by ITT Cannon, and already in use on Teletype terminals and IBM mainframes, which is probably why they ended up in the IBM PC. As has been pointed out, unless there's something exotic in the circuitry, the standard 9 pin connectors will probably do. If all you want is a rack-mount PC, you can get something surplus for a few $100 like this one from Weird Stuff.
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Re: RS232 (serial) ports- DB25 vs DB9

#9 Post by Microspeed » Sat Jan 08, 2011 11:43 pm

I have had some experience building computers to work with some proprietary systems. One particular time I mistook a DB9 connector for an RS-232. It turned out to be an RS-423. What I really needed the correct I/O card. The desktop I was replacing had a custom I/O setup; I just replaced it with a PCI card. It was still cheaper than buying the box from the machine manufacture, just took a little more work. What I am trying to say is this; when dealing with proprietary controls be sure of the specs. I found a site that has some information that might be useful.

http://www.lammertbies.nl/comm/info/RS-485.html

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Re: RS232 (serial) ports- DB25 vs DB9

#10 Post by virge » Sun Jan 09, 2011 8:12 pm

I've just gotten my hands on a PC that was retired (failed) off the line. I have it opened up and it appears to use an off the shelf motherboard. I don't see anything proprietary at all. It's got a 10GB HDD with Win98, CD-ROM, and floppy. Standard ATX power supply, all in a rackmount case. The software comes separately on a couple floppy drives. I just don't see anything to justify the $2k per machine.

One thing I noticed is that the DB25 serial connector plugs into a 9-pin header (COM2) on the motherboard. Logically, this seems to mean that only 9 pins are being used... therefore a DB9 connector would just fine. Is that right?
bill bolton wrote:Unless the process control kit you want to interface to is extremely old, you shouldn't have a problem.
I will find out about how old this equipment is. I was under the impression that it's about 15-20 years old.

Thanks for the links and all the info. This very, very helpful for someone like me who's only prior experience with serial ports was using serial mice when I had an AT computer. Obviously, not nearly as old as some fellow thinkpadders. :lol:
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Re: RS232 (serial) ports- DB25 vs DB9

#11 Post by Microspeed » Mon Jan 10, 2011 12:32 am

It sounds like you might be well on your way. The only other thing I could suggest is a lot of the control programs written back then for Win 98 were very picky about the version of both the OS and more importantly the version of the msvcrt.dll. From what I understand this dll is used by programs writen in C++. The problem you run into is that so does Microsoft and other programs present on the system.
So what does that mean? Your control program will suddenly not have all or only part of certain drop down menus.
From what I understand this is the basic rule of thumb. When Microsoft uses an address within that dll they “Lock-it”. Another program cannot use that address or move the Microsoft claim on it. This is why you need the same version of Win 98 with the same version of MSVCRT.DLL
The other kicker is that any other program, Direct to CD is one that comes to mind, will write to and change the version of the MSVCRT.DLL and that will cause you errors. I would recommend that you only load the same programs that are present on a working machine.

Good luck,
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Re: RS232 (serial) ports- DB25 vs DB9

#12 Post by virge » Mon Jan 10, 2011 12:45 pm

Thanks for the tip about WSVCRT.DLL. I'll make sure not to install any extraneous programs. In fact, if the old hard drive is still good, I'll make an image of it. From past experience, it seems that Win98 can handle being moved from machine to machine a lot better than XP.
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Re: RS232 (serial) ports- DB25 vs DB9

#13 Post by mediasponge » Mon Jan 10, 2011 6:41 pm

virge wrote:Thanks for the tip about WSVCRT.DLL. I'll make sure not to install any extraneous programs. In fact, if the old hard drive is still good, I'll make an image of it. From past experience, it seems that Win98 can handle being moved from machine to machine a lot better than XP.
Biggest issues I had moving Win98 from old machine to new machine were either video card drivers, or IDE controller drivers. The video will revert to Standard VGA while you sort out the drivers, but the chipset/IDE drivers are a bigger issue. While most machines will revert to a default IDE driver good enough to boot from the HDD, there were times that the machine would not see the CDROM and I had no way to update the drivers because they were on CDROM! Bottom line, put the drivers for the new mobo and video card on the HDD before migrating it to the new machine, even if that means plugging it into another machine as a slave only to copy the contents of the CDROM over. You can make a C:\i386 or C:\drivers folder, and dump everything in there from the MS Win98 CD and each of the hardware suppliers. Same goes for the NIC, if these machines have one. That way, when the machine boots up and asks for a driver, you can point it to one on the HDD.
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