OT (again): MODEM to MODEM?

Hello Nick:

The "null-modem" cable is just a serial cable with one set of wires crossed over. It works by connecting the serial ports of two different computers together, _without_ a modem.

I may have a few parts that you could borrow (including a null modem cable). Geographically, I think we are "close". Send me an email if theres anything I can do.

Wayne

You would be much faster with a parallel cable or a USB. The P-cable is cheaper and the OS should handle it well. The USB needs a special cable and they are about $15. Another way is to use a CAT 5 with a crossover and plug them into net ports or just get a cheap hub and go network. Serial to serial needs a nul-modem or a crossover and it's slow. Never heard of hooking 2 modems together. The question is: What are you trying to do and why?

Yes, specifically the transmit and receive pins.

The RS232C spec (serial connection specification) has two kinds of equipment in mind. Data Transmitting equipment (such as a modem), and Data Receiving equipment (such as a PC). The main difference between these two is which pin they talk on, and which pin they listen on.

If you hook two of the same type together directly (like two computers, or two modems), they don't communicate, as they are both talking on the same pin, and listening on the same pin, so neither hears the other. When you hook one of each together (like a computer and a modem) they work fine, as one is talking on the pin the other listens on, and vice versa.

A null-modem lets you connect two of the same kinds of equipment together because it crosses the transmit and recieve wires. This connects one piece of equipment's transmit pin to the other's recieve pin, and vice versa. All the other wires, such as the chassis ground, signal ground, ring, etc. are straight through connections. Only the transmit and recieve wires are crossed. If memory serves, that's pins

2 and 3, but don't quote me on that.

I know you can connect two PC computer serial ports together with a null modem, I've done it many times, mostly back in the MS-DOS days to move data between machines. Just make sure that the port settings are compatible on both machines...mostly the baud rate, parity settings and byte size (8 bits, no parity, and 115,000 baud works well). Make sure that whatever software you are using to do the communication is set to use the comm port you have connected too... ;-) There's no need for any modem communication stuff (ATDxxx) on such a connection, and it's possible that having it there might confuse the other machine, so tell whatever software you are using that it's just a direct serial connection, with no modem.

Whether you can get two modems to talk over a piece of phone wire, without a phone company in the loop, I can't say. It will probably depend on how the two modems are designed. If they aren't using any of the power available on a typical phone line for anything, and can be told to go on-line without dialing or having a dial tone present, it might work. If not, it won't.

-- Mike B.

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There are several ways to skin that cat and you were told of several of them. Two modems will talk to each other without any phone company stuff in the way by the method you were told. Better tho is to not even bother with the modems and just use a null modem cable between the computers. The cable swaps the TX and RX pins and also swaps some other pins that detail the handshaking between the two computers so that the serial ports will transmit their data as it is sent to the ports. I'll note that serial port communications has been made more complex by the several schemes that various idiots wanted to do to "insure" that the data was sent properly. Unfortunately, the idiots all decided on different methods to do this so we now have at least a dozen different standards of handshaking requirements to get things moving properly.

-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!

Probably... Depending on your particular modems.

That would be if you want to eliminate both modems and connect your two test PCs together directly. It would simulate all parts of your connection except getting the modems talking.

There is also "ato" which causes some brands to go "online" without needing a dialtone. Back in the 300 baud days I know I was able to connect two modems with a phone cable (no telephone company CO between them) and have it work. Haven't tried it since then. Or with Aussie equipment...

There are several ways you could arrange the connection. Simplest would be buy PC Anywhere, which knows about modems and could do your job without any IP addresses or networking jargon. It is also the only option I know of that will be tolerably speedy over a dialup modem link.

I'd say the next level up in complexity would be to each connect to your normal internet connection (assuming it is a real ISP and not AOL or something proprietary). You disable any firewall software on your dad's PC. Your dad runs something like IP2 to find out the "dynamic IP address" his system got when he dialed up. You then use that IP address in your remote control software to reach his PC via the internet.

(Of course his dynamic IP will only stay the same until he hangs up that connection. If he has to hang up to call you to tell you the dynamic IP, he loses it! People solve this by setting up a "chat" connection or eMailing the dynamic IP to each other.)

Making a private modem-to-modem IP connection seems to me to be the hard way to do this. You need to change so many things from their normal internet configurations. And neither of you can use the internet while you connect to each other.

You do lose this if you connect via the internet (unless you happen to both have V92 modems, which are designed to do this...) Maybe you could use an IP phone service like Net2Phone, or a "chat" system to talk?

If you do want to continue with this route, try connecting your modems with a phone cable and using "ato" and/or "ata" - might work. Or use the null modem serial cable to connect the two PCs without the modems, which _will_ work.

Loren

Am I right in thinking the purpose of this is remote control of you dad's PC for troubleshooting?

I know this isn't what you asked, but here's how I'd do it:

Get him set up with an internet connection if doesn't have one already. I like adial.co.uk, it's as easy as it gets. The only cost is the local-rate call when online.

Get a VNC server set up on his machine. TightVNC is good for low bandwidth (modem) connections. It's free.

Get a no-ip account. This will give him an address like mydadspc.no-ip.org, so you don't need to get him to find out his IP address. There is a little program which automatically updates the mydadspc.no-ip.org to point to his IP address every time he connects. The basic service (all you'll need) is free. Set the account up before you go, so it's ready when you get there.

VNC isn't the most secure thing in the world, so you might not want to leave it running all the time (you can install it so it runs all the time). Put an icon on the desktop to start it if you're worried about security. Then all he has to do is connect to the 'net and click the icon. You start your VNC client and connect to mydadspc.no-ip.org and you have control of his PC.

You'll still have the problem of testing after you've set everything up, but as all the testing involves is starting the VNC client with the right address (and password!), which can be done from any other 'net connected PC, you should be able to get your SO or a friend to do that while you're still at your dad's (then change the password!).

Tim

Good one Nick, I'm not sure (says a 25 year telecom guy). To see if you can connect the analog (phone) side of two modems back-to-back, the place to ask would be within the comp.dcom.xyz newsgroup hierarchy, specificly comp.dcom.modem or comp.dcom.modems (I just looked at the list and both are there, not sure which one is active, maybe both ??). There should be a way, as many modems can be used on 'leased lines', which is a point-to-point circuit with no DC bias (? 'ring voltage' ?) or dial-tone (the circuit isn't switched so no dialing required). Anyway, there are many cheap modems that can not be used in this mode, but many that can. As far as specifics, I'd ask in the comp.dcom.modem(s) group.

If you just want to simulate a modem connection, without using modems, you need a real 'null modem' (as opposed to a null modem cable). Have a look at

and search on 'modem eliminator', you'll find a little box and also an explanation of their function. They provide the handshaking that a modem would otherwise do (something a simple cable cannot do), and also provide clocking in synchronous applications.

Good luck with your project. If I can be of any assistance drop me an email. Note that there is only one 'w' in 'shaw' , and it should be .ca , not .ceh .

Dave

Personally..Id not bother with a modem at all..and simply install a couple of NIC (network interconnect cards) which are about $10 each and either a peer to peer cable or a $10 hub and two standard network cables.

Set up your software as a simple network, and voila..you have full access to both computers at far greater than serial speeds. This takes about 5 minutes to do the software setup in Windows, and works fine.

I use this method now exclusivly and no longer have to futz around with PcAnywhere, or LapLink etc etc.

Ive not tried the USB method, so cannot comment on it.

Gunner

"At the core of liberalism is the spoiled child - miserable, as all spoiled children are, unsatisfied, demanding, ill-disciplined, despotic and useless. Liberalism is a philosphy of sniveling brats." -- P.J. O'Rourke

In article , Mike Bartman wrote: : :A null-modem lets you connect two of the same kinds of equipment :together because it crosses the transmit and recieve wires. This :connects one piece of equipment's transmit pin to the other's recieve :pin, and vice versa. All the other wires, such as the chassis ground, :signal ground, ring, etc. are straight through connections.

That's not true. There are several status signals that must be cross connected if you want anything resembling the common, old communications software (Kermit, Wincom, Minicom, Zmodem, etc.) to work:

Outputs: Data Terminal Ready, Request to Send Inputs: Data Set Ready, Clear to Send, Ring Indicator

If you just wire those straight through, you'll have outputs fighting other outputs, and pairs of floating inputs connected. No matter what you do, you cannot exactly simulate what the software will see when talking to an actual modem. The hardware just won't allow it.

On the original question about connecting two modems together -- Most modems today use an electronic relay to connect to the telephone line. You'll need to supply a DC bias current to allow those components to operate properly. Without that bias current your connection will be intermittent at best. Been there, done that, got tee shirt.

Three wires are all you need for software flow control or no flow control, the only reason for the others are for hardware flow conrol. Quite often you can get away without flow control on todays fast computers.

On Mon, 06 Sep 2004 14:16:47 GMT, "Wayne Bengtsson" vaguely proposed a theory ......and in reply I say!:

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Wayne. Thanks for the offer. But I do actually want to simulate the whole shebang, if I can. In the end I will be back on modems, so I want to be sure!

It had just never occurred to me to "simply" connect two modems as is.

***************************************************** I know I am wrong about just about everything. So I am not going to listen when I am told I am wrong about the things I know I am right about.

On Mon, 06 Sep 2004 15:55:58 GMT, "Tom Gardner" vaguely proposed a theory ......and in reply I say!:

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In the end I will be away again and have to use the phone lines. So although I know there are heaps of better ways, that's what I want to set up, as close as the real thing as possible, without an hour's drive each way when it falls over

***************************************************** I know I am wrong about just about everything. So I am not going to listen when I am told I am wrong about the things I know I am right about.

On Mon, 6 Sep 2004 10:04:01 -0700, "Bob May" vaguely proposed a theory ......and in reply I say!:

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BTW. Eating less does not always work. It depends how much too much you were eating before!

OK. I will give this a go with a phone cable when I get two PCs and modems near each other.

Again, I want to include the modems, just to get the whole chain working, although it's not best by far.

Crikey! I can get a _network_ card for $10 these days, if I want connectivity!

***************************************************** I know I am wrong about just about everything. So I am not going to listen when I am told I am wrong about the things I know I am right about.

On Mon, 06 Sep 2004 10:27:23 -0700, Loren Amelang vaguely proposed a theory ......and in reply I say!:

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Well at least it's worth a go.

OK. Which I don't want here.

OK. Yep. It slowly comes back. Need to grab an AT command set.

Loren. Yeah. PCAnywhere. I certainly remember it being a lot simpler than this when I used it years ago under DOS, when I was supporting remotely. It takes control, whereas many of the others (RADMIN, VNC) require you to set everything up, and they are basically front-ends. I just find it hard to justify the cost of PCAnywhere for this exercise.

Also, RADMIN, which I have used is very fast. I have seen comparisons that place it above PCAnywhere.

He will NOT use the Internet. Sorry.

Yep. DCone that sort of stuff in the past over the Web.

That's fine.

I have made this work before. It's just that it's frustrating, and I was looking for a way to "be two places at once" .

I am going to try the modems first, and thanks.

***************************************************** I know I am wrong about just about everything. So I am not going to listen when I am told I am wrong about the things I know I am right about.

On Mon, 06 Sep 2004 17:53:21 GMT, "DaveK" vaguely proposed a theory ......and in reply I say!:

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Interesting info, and a brave admission

I will follow up if needed. Thanks.

***************************************************** I know I am wrong about just about everything. So I am not going to listen when I am told I am wrong about the things I know I am right about.

On Mon, 06 Sep 2004 13:06:24 -0700, JR North vaguely proposed a theory ......and in reply I say!:

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Yes. I know PCAnywhere from Wayback (nice town that). It certainly did the job very simply that I remember. But $$ not justified here.

***************************************************** I know I am wrong about just about everything. So I am not going to listen when I am told I am wrong about the things I know I am right about.

On Mon, 06 Sep 2004 21:46:32 +0800, Old Nick vaguely proposed a theory ......and in reply I say!:

remove ns from my header address to reply via email\

Crikey! Avalanche!

Thanks guys. See replies. And I reckon you've all been very nize!

................sorrry...

***************************************************** I know I am wrong about just about everything. So I am not going to listen when I am told I am wrong about the things I know I am right about.

How far apart are the two ends?

A null modem simply allows direct connecting of two RS-232 ports together, exchanging pins so the right signals wind up in the right places. (w.g. pin 7 on a 25-pin serial port is common signal ground at both ends. pin two on one end connects to pin three on the other (TXD and RXD (transmit data and receive data)). The 9-pin ones use different pin numbers, and I don't remember what is what among those)

There are several handshaking signals which also have to be exchanged. This is what a "null modem" does. Just two connectors and wires appropriately connected. They are the minimal cost, and thus the way of choice for close systems.

You still need some kind of protocols between those to make them pretend to be ethernet. These *may* be built into the computer's OS, but they will have to be selected in some way.

However -- depending on data rate, these null modem cables are only good for perhaps 25 feet (at maximum data rate) to perhaps 100-200 feet (at a very low data rate). From what you say elsewhere, this is not sufficient for you.

Sure -- this works, but for close systems, it costs a lot more than a cable with a connector at each end (null modem), so it does not make sense.

Nobody is. Both are telling you what makes sense for their understanding of what you are asking. The protocols are needed in either case. The modems or the null modems.

The modems have the advantage that (over the proper kind of wire setup) they can carry the data over a much longer distance. However, if you are talking about the two ends being an hour's drive apart, where do you get the wires between the two ends? Here in the USA, the options are to dial up between two phones as the modems are normally used, or to lease a pair of wires from the phone company (and they do *not* make this cheap. :-) With an hour drive, there are probably booster amplifiers in the phone lines (or often, at the exchanges which they pass through), and I don't think that you can make a pair of modems do the job without the boosters.

Sure -- and the protocols (PPP or SLIP) are needed to pump ethernet formatted data across a modem connection.

Of those 23 wires, usually only about eight or ten are used to talk to the modems at the most. And really, all you need are three wires, if you don't have to deal with older style modems which had a fast direction and a slow direction, and needed control signals to swap which direction was fast. (And "fast" was only 1200 baud, with the "slow" end something like 50 baud. :-)

[ ... ]

And it is *this* one which makes me wonder where you are getting the wires if you *don't* use a pair of modems and a dialup connection. :-)

[ ... ]

You don't *need* an ethernet work-alike for file transfers. Look up kermit -- get an older version, from back when the PC version was free. You can do an amazing amount over that -- as long as you are willing to do everything by typing commands, and don't need a GUI. If you need a GUI, you will proably also need something to emulate ethernet.

If you take the time to tune the options, you can get much faster file transfers -- as long as the noise in the phone line is not too bad. (Each burst of noise forces a retry of a packet of data, and longer packets are more efficient if you don't have many retries, but short ones do better in the face of noise.

And -- that will probably get in the way of the ethernet emulation.

This would so much easier if everything were in the same house. That hour's drive is a killer.

Good Luck, DoN.