What is flow control, and how do I set my modem to use it?

If your modem runs at 2400bps or faster, and has built-in error correction and data compression, then something must control the flow of data between your modem and your computer. Whether hardware- or software-based, flow control is what prevents your computer from being flooded with too much data too fast.

Software flow control is called XON/XOFF; hardware flow control is called RTS/CTS Control (Ready to Send/Clear to Send). You can use your communications program to set both kinds of flow control (you can usually find the flow control options in the Setup menu). You can also use the instructions below to set flow control in Windows.

If your modem is fast enough to need flow control, use RTS/CTS. XON/XOFF sometimes injects unwanted characters into binary files as you upload or download them, so you should use it only when communicating with another computer that demands it.

Windows 2000 and XP

Follow the instructions below to change your flow control settings:

1. From the Start menu or the desktop, right-click the My Computer icon and select Properties.
   
   
2. In the Properties window, click the Hardware tab, and then Device Manager... .
   
   
3. In the Device Manager window, click the + (plus sign) next to Modems. Right-click your modem, and then select Properties.
   
   
4. In the Properties window, click the Advanced tab and then click the Change Default Preferences... button.
   
   
5. You should now see a list of several options. Flow control should be the last option in the list. Configure it as you wish.

Windows 95, 98, Me, and NT

Follow the instructions below to change your flow control settings:

1. Click the Start button and select Settings, then Control Panel.
   
   
2. In the Control Panel window, double-click the Modem icon. Make sure that your modem is highlighted, and then click Properties.
   
   
3. In the Properties window, click the Connection tab, and then choose Advanced.
   
   
4. You should now see the flow control options available. Edit them as you wish.

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