If you've never heard of Synergy, check out there website here.  In a nutshell, it's a program you run on two computers, which lets you use the keyboard and mouse of one to control the other.  Think of it a little bit like a KVM switch in software.  The connection is made over your network or the Internet.  To switch machines you just slide your mouse off the side of one screen and onto another, as if you just had a second monitor set up.

Recently at my day job, I used Synergy to connect my main work computer to a spare computer I set on my desk but did not want to make room for another keyboard and mouse.  I also did not want to connect our company network to the spare computer, for security reasons.  Now, I could have just used a cross-over ethernet cable, but did not have a spare ethernet port on my main computer.

Enter my savior: an ancient serial Null Modem cable.

In the good old days, these little serial cables were used to connect two computers together at a relatively low speed for file transfers and the like.  And if you happen to have one laying around (possible with an extra "null modem" block attached, possibly not), then this guide will help you set up two Windows computers to use it to work with Synergy.

The Cast of Players

• Main Computer - this is my primary computer whose mouse and keyboard will control the other one.
• Spare Computer - this is the name of my spare computer.
• Windows XP - The operating system running on both, though I assume the steps are very similar for other versions of Windows.
• Nully - The null modem cable.

We will begin by creating our network connection between the two computers using the null modem cable.  We'll do this before installing Synergy, so be sure to leave your mouse and keyboard connected to the Spare.

 Step One - Setting up the host

Begin by physically connecting your null modem cable between both computers.  Next, we will set up the Spare computer to act as the connection host.  The Main computer will "dial in" to the spare to form this connection.

1. On the Spare computer, go to your Network Connections screen (Start / Control Panel / Network Connections).
2. Click "Create a new connection." 
3. Choose "Set up an advanced connection." 
4. Then, "Connect directly to another computer."
5. For the role, select "Host."
6. For the device, it is probably going to be COM1 or some other COM, depending on where you plugged in your cable.  If all of this fails to work, you will need to do a little experimenting here, and possibly select other options.  On my setup, I select COM1.
7. On the next screen, you specify which users are allowed to connect.  The easiest thing to do is just click "add" to create a new user.  Give it the name of "main" and a password of "main", or anything else you want.  This doesn't have to be terribly secure, since it is unlikely anyone will be hacking into your computer from the serial cable!
8. Click Finish.  You should now see an "Incoming Connections" icon.  Right-click that and choose Properties.
9. On the networking tab, highlight "Internet Protocol TCP/IP" and click Properties button.
10. On the screen that comes up, click that you wish to specify IP addresses.  Since we only need two, I entered:
       From:  192.168.1.50
       To: 192.168.1.51
    In reality you can enter almost anything you want.  I suggest you enter a range which will not interfere with any other IP's your computer may be using.
11. Be sure to check the "Allow calling computer to specify its own IP address".
12. Click OK, and you're done setting up the Host!

Step Two - Setting up the client

Now that we have the Host set up on our Spare computer, we will do something similar on our Guest computer, which is our Main computer.

1. On the Main computer, go to the Network Connections screen and click to add a new connection.
2. Like before, select "Set up advanced connection", then "Connect directly", but this time click "Guest."
3. Enter the name of the spare computer for the Computer Name.  I don't believe this has to be exact; I think you can enter anything you want.
4. Select your device; again, probably a COM port.  And again, if this fails, you will probably need to experiment here with selecting any other ports which are listed.
5. Click finish.  A new icon should appear, named whatever you called the connection.  Right-click it and choose properties.
6. On the Networking tab, highlight Internet Protocol TCP/IP and click Properties.
7. Check "Use the following IP address" and enter 192.168.1.51 (or whatever you specified in the To: field when setting up the host's ip range).
8. Do not enter anything for DNS server addresses!  Click the Advanced button.  Uncheck the "Use default gateway on remote network" checkbox.
9. Click Ok to save everything.

Alright, when you get back to the Network Connections window, you can double-click the connection icon.  It will prompt you for the username and password you selected before when setting up the host (I chose main/main).

Cross your fingers, and hope it tells you that it worked!

Uh oh-- problems

• Didn't connect! - You possibly selected the wrong COM port on the device selector, on either the Host or the Guest.  Unfortunately this can be kind of a guessing game, and you just have to keep trying different combinations until you get it.  Unless you plugged your cable into a very wide port, don't select LPT as the port; that's the printer port.  If you plugged into a smallish, 9-pin port, that is a COM port.
  • Another problem might be some sort of IP conflict.  This can be resolved by going to the host and entering a different range, and then updating the specified IP address on the guest.  Beginning with 192.168.1.X is probably best, you can also try 10.10.X.X numbers.
• Internet isn't working on main computer - This is possibly because your Main computer is now trying to get to the Internet through the null modem connection.
  • Did you remember to do item #8 on Step Two above?

Load connection on start up

Once you have confirmed that everything is working (including the Internet on your Main computer) you will probably want this connection to load on start up.  On the Main computer, right-click the connection icon and select Properties.  Go to the Options tab.  At the top, under Dialing Options, you can uncheck the "Prompt for name and password" checkbox.  Click Ok.

Now, right-click the connection icon and say Create shortcut.  It will place it on your desktop.  You can now drag that to your start button (hovering for a second), then to All Programs, then to your Startup folder.

 Now for the meat and potatoes - Synergy!

Compared to what we've just set up, this is now the easy part.

The first thing you want to do is confirm that your Spare computer can "see" your Main computer.  On the Spare computer, go to Start / Run.  Type "cmd".  In the window that opens, type "ping 192.168.1.51"  (the IP address for your Main computer you selected).  You should see replies come back.  If you get "Request timed out" then you have a problem, and must start over!  Make sure you entered the correct IP address for the Main computer.

Now, go to Synergy's website and download a copy on each computer and install.  If it doesn't want to run for some reason, try getting an earlier version.

Run Synergy on both computers.

On the Main computer, select 'Share this computer's keyboard and mouse (server).'  You will then need to click 'Configure' to set up the Screens & Links.

This screen may seem a little weird.  The idea is you are telling Synergy which screen is to the left or right of the Main computer.  Your screen names must be exactly what each computer is named!  I will attach a screenshot of my screen for clarity:

Once you have your particular setup configured, click "Test" on the main window to begin the server.

Now, on your Spare computer, run Synergy, but this time check the "Use another computer's sshared keyboard and mouse (client)" and enter "192.168.1.51" for the other computer's host name (or whatever you selected for the Main computer's IP address).  When you're ready, click Test.

Be patient.  Sometimes setting up this connection over a null modem connection can take 10 - 30 seconds.  If it worked, you can set Synergy to "AutoStart" on both computers, which is what I would recommend.

It works-- but the mouse is slow and lagged on the Spare computer!

If this happens, it means you need to adjust the speed of your COM ports on BOTH computers.  To do it, go back to your Network Connections windows on both, and right-click the connection icon and select "Properties."  Select your COM device and click Properties or Configure.  On the next screen you can change the speed.  I set mine to "115200" for both.

Congrats!

I sincerely hope everything worked!  I know this is kind of a lengthy procedure, and there are a couple places where something could go wrong.  Best of luck!  If this worked for you (or if you found an alternate way of doing it) please post a comment!