I’ve been into computers since around the time DOOM came out. I still play that game to this day, and now that I have a few computers laying around the house, I thought I’d try to figure out the “Null-Modem/Serial” feature that was available in some early 90’s multiplayer games (DOOM, Descent, and I’m sure other current games too).
I tried to find info on it, but every page I’ve been to assumes you already know what the hell a Null Modem is. I have deduced it is some sort of cable that connects serial ports between two computers. So I went out and got the only cable that I thought fit the bill (two DB9 female connectors). I tried to hook up the two machines and start a null modem game but they didn’t seem to be communicating (they just sat there waiting for an opponent).
Any info anybody can supply is greatly appreciated. Believe it or not, it didn’t require any null modem knowledge for me to get my CIS degree.
It’s been a while, but IIRC, a null modem cable is just RS-232 with pins 2 and 3 crossed (pin 2 at one end connects to pin three at the other). Right now you have the data in and data out lines connected to each other.
If that isn’t right, I’m sure somebody will correct me. Or post back and I’ll actually look it up.
The simplest null modem cable crosses the transmit and receive lines.
A standard cable that just has two female ends will not cross the transmit and receive (pins 2 & 3 on a standard DB25 connector, IIRC), all the lines just run straight through.
Standard RS-232 wiring specifies two different kinds of equipment, DTE and DCE. DTE (Data Terminal Equipment) is what the serial port on your computer is wired as. DCE (Data Communication Equipment) is what your modem or a serial printer is wired as.
The difference between the two is which line transmits data, and which line receives data (and a few other functions which have the equivalent of “send” and “receive”.
If you run the lines straight through from DTE type equipment to DCE type equipment, everything works OK. The “transmit data” line on the computer goes straight through to the “receive data” line on the modem (and so on with the other signal types). This is what normal cables do.
But if you try to connect two DTEs (two computers), you have to “cross over” the transmit line on one to the receive line on the other.
There are several other signal lines that can be crossed on a null modem (DTR to DSR, RTS to CTS, etc.) that support hardware handshaking. Most standard null modem adaptors or cables will probably have these lines crossed as well.
The cable you bought was probably just a straight through cable with two female ends. This is the kind of cable you would use to connect a piece of equipment wired as DTE to a piece wired as DCE, such as a computer to a printer.
So you need a cable or adaptor that specifically says “Null Modem”, or the appropriate lines aren’t crossed.
you can use hyperterminal to communicate directly with the serial port. Once you can type on one keyboard and see the characters on the other screen you are in business.
Thanks for all of the info (esp the technical stuff, RJK: why the hell couldn’t I find an explaination like that online?).
It’s funny you bring up Radio Shack Arjuna, as they’re specifically mentioned in the DOOM README.TXT (“go to Radio Shack and tell them you need a Null Modem cable to play DOOM”). Of course, that was written years ago. My brother was going to go run some errands, so I told him to stop by Radio Shack and see if they had the cable. Just in case I gave him my best guess as to what one was (the two female DB9 cable). The person at the Shack didn’t know what DOOM was, didn’t know what a Null Modem cable was, and had no idea what DB9 meant.
Well, tonite I’ll attempt to turn my cable into a null modem one. But since I have cards tonite (sheepshead of course), it may not get done, in which case I’ll use some of the great links provided.
Anyway, you’re welcome. Basic though this stuff is, unless you actually play around with it, it’s not as simple as it might seem to get a handle on.
Since I write programs that (among other things) sometimes have to communicate across RS-232 lines to various kinds of hardware, I had to get at least marginally familiar with the process. It took a while to sort it out and make some sense of the whole thing based on the skimpy info most vendors give you about their products.
I’m not the least bit surpised I usually have to grab one of their catalogs in the store and find the part number, so they can look it up in the computer. If the computer tells them they’ve got one in stock, they can somehow find it for you. Otherwise, you’re on your own. Any further talk of Radio Shack will have to be done in the Pit!
Sorry to post twice in a row, but I had to comment on this:
And it’s a sad but all too true state of affairs.
I grew up going to Radio Shack to get parts to build & repair various bits of electronic equipment. Over 30 years of going there, and it’s gotten steadily worse, especially over the last ten or so years.
A few years ago I needed a transistor, so I wandered into a Radio Shack (even though I knew their inventory, not to mention expertise, has been shrinking in that area) and asked the kid behind the counter if they had a general purpose, PNP, medium power transistor.
He looked at me like I was speaking Martian. I tried again, and asked if they had any transistors at all. He still looks confused, so I say “You know, transistors. Little metal parts you see on circuit boards”. Still looking confused (but sensing something) he leads me over to a corner in the back where they have some resistors and a few caps, and a couple of ICs. Close, but no cigar.
Ok, it bugged me they didn’t have any transistors at all. But what struck me as worse was the fact that the person behind the counter in a Radio Shack (you know, Radio Shack, named for a place that had parts to build radios) didn’t have a clue what a transistor was.
These days, if you’re not looking for a cell phone, video game, or the like, you better know exactly what you want. Cause most of the folks behind the counter have no clue at all.
I usually have to grab one of their catalogs in the store and find the part number, so they can look it up in the computer. If the computer tells them they’ve got one in stock, they can somehow find it for you. Otherwise, you’re on your own. Any further talk of Radio Shack will have to be done in the Pit!