ZyXEL Elite 2864I User Guide - Page 222

For high speed serial connections at 230.4Kbps or 460.8Kbps, use a low-capacitance cable. Also, keep the cable as short as possible. Parallel Cable Use a 25-wire cable with a male connector at one end and a female connector at the other end to connect the PC's parallel port to the modem's parallel port. Keep this cable as short as possible. This cable may also be used to make the 25-pin serial connection. Using the Windows 3.1 Terminal Program You will not be able to use a transfer rate of more than 19.2Kbps with this program. With a special comm driver, you may be able to run the serial port at a higher speed. Please consider this when you set up your modem's DTE to DCE speed. If you are using a ZyXEL 2864 modem's parallel port for PC-modem communication, setting the DTE speed has no actual physical meaning. The actual speed depends on how fast and efficient the Windows program can run. The parallel connection guarantees no data loss, no matter how fast or slow the Windows program runs. Fast 286, 386, 486 and Pentium PCs allow you to lock the serial port at speeds above 38.4Kbps. Check whether your PC and software allow this. Refer to the ZyXEL Serial/Parallel I/O Adapter Card section in this chapter for information about high-speed connections. Hardware Hints The serial port is driven by interrupts. Every interrupt needs a certain amount of overhead processing time. Too many interrupts reduce the computer's efficiency. The UART 16450 is very commonly used in serial port devices. For every character (byte) received, it generates an interrupt. If your hardware allows it and if your software supports it, replace the 16450 UART with a 16550 model. This newer chip has an internal buffer and generates an interrupt for up to every 16 characters (several trigger levels are available). With this UART installed, you may drive your serial port at 57600 bps and above. Among other programs, the shareware terminal program Terminate allows you to determine which type of UART(s) is/are installed in your system without opening the computer. If you run Windows, OS/2 or another multi-tasking operating system, you should set your serial port speed to 38400 bps or less. In these environments, the serial port is usually served slower than in a single task environment. Another option in multi-tasking environments is to increase the priority of your serial port. If you use Windows, read the file SYSINI.WRI. This file is placed into the 21-2