National Instruments GPIB-232CV-A User Manual

Contents

National Instruments GPIB-232CV-A User Manual Manual pdf 30 page image

Chapter 3 Operation of the GPIB-232CV-AGPIB-232CV-A User Manual 3-4 www.ni.comChoosing a Serial Port Buffer SizeAll data that is sent to the GPIB-232CV-A is buffered and then passedalong to the destination port. Thus, any serial data received is buffered andthen passed along to the GPIB port whenever the GPIB-232CV-A isaddressed to talk. The GPIB-232CV-A has 256 KB of RAM for buffering.In D mode, this memory can be allocated between the serial and GPIBbuffers in two possible configurations.The size of the serial input buffer can be set to either 256 bytes(SW1 switch 7 set to OFF), or 32 KB (SW1 switch 7 set to ON) with theremainder of the 256 KB of RAM allocated to the GPIB buffer.• Set the serial input buffer size to 256 bytes if only small amounts ofdata, such as status information from a plotter, are expected to be sentfrom the serial device to the GPIB-232CV-A.• Set the serial input buffer size to 32 KB if you expect the serial deviceto send large amounts of data to the GPIB-232CV-A, such as anRS-232 instrument sending thousands of bytes of measurements.In either case, the remainder of RAM space is used as the GPIB inputbuffer. Thus, if a large amount of GPIB data is expected to be sent to theGPIB-232CV-A, it would be advantageous to use the small 256 byte serialport buffer size if possible. Then the majority of RAM is used as the GPIBbuffer. The larger serial buffer size is a means of capturing a largerpercentage of the total RAM space for serial input use.Regardless of the buffer size you choose, serial data is not lost because theGPIB-232CV-A implements a hardware handshake scheme, and ifselected, the XON/XOFF software handshake scheme. The variable buffersize is a means of optimizing the GPIB-232CV-A for your application.Using the SRQ FeatureThe GPIB bus performance might slow down if the Controller is constantlywaiting for the GPIB-232CV-A to send or receive data. With SRQ enabled,the bus can be used by other devices until the GPIB-232CV-A requestsservice (asserts SRQ*). When the GPIB-232CV-A asserts SRQ*, theController serial polls the GPIB-232CV-A to determine what type ofservice it needs. After it is serial polled, the GPIB-232CV-A unassertsSRQ*. You can enable the SRQ feature by setting switch 6 of SW1 to theON position.