Data Acquisition

The need for PCs to exchange data with remote instruments and distributed or stand-alonedata acquisition systems over long distances over the existing telephone network and tofunction as intelligent terminals, has furthered the development of various serialcommunications adapters. A standard PC has two asynchronous serial communications portscalled COM1 and COM2. DOS supports up to four such ports, and some PCs may also havea COM3 and COM4.The electrical characteristics of the ports conform to published Standard RS232 and theyoperate at speeds from 50 bps to over 115.2 kps. IBM compatible PCs are typically fittedwith an RS-232 serial communications card with two serial ports.4.9.1 Standard settingsI/O address of the serial ports:Serial port 1 (COM1) 03F8 HexSerial port 2 (COM2) 02F8 HexWhere additional serial port hardware is fitted these ports have the following settings:Serial port 3 (COM1) O3E8 HexSerial port 4 (COM2) 02E8 HexHardware interrupts:Serial port 1 IRQ 4Serial port 2 IRQ 3For communication over an analog telephone line, a modem is connected to a serial port, orit may be incorporated in an add-on board. A modem converts the digital data generated bythe PC into a signal that can be transmitted over an analog line. Some modems incorporatedata compression and error correction techniques to achieve much higher transmission ratesthan the bandwidth of a telephone line would normally support.The COM ports are also used to attach terminals to multitasking, multi-user environmentssuch as UNIX; this often requires multiple COM ports. Add-on boards are available whichsupport two or four COM ports; further COM ports may be added by using multiple 8-portCOM boards. DOS will not support COM ports higher than 4, but the hardware itself isexactly the same as that for ports 1 to 4. The additional COM ports appear at different I/Ochannel addresses and may be accessed directly by the application software.4.9.2 Intelligent serial portsAn intelligent serial port is designed to relieve the load on the main CPU of the host PC. Ittypically contains an Intel 80186 microprocessor with read only memory (ROM), containingan operating system and some RAM. When the board is initialized, the operating system isloaded into the RAM together with any other user programs. There is also dual ported RAMon the board for transferring information between the PC CPU and the on-board RAM. The80186 then polls each of the serial ports on the board to read for any incoming data or totransfer any outgoing data. The user programs contained in the RAM can also pre-process thedata before it is transferred to the PC system memory. The transfer is accomplished either byDMA, which does not use much memory, or by memory-mapping the dual ported RAM intoPC memory, 64 kBytes of PC system memory.