Data Acquisition

initiated. The data is subsequently read and transferred to the PC’s memory. This incurs alarge software overhead. Background operation using interrupts is difficult and slower thanpolled I/O and accurately timed samples and higher speed data transfer methods such asDMA and repeat instructions are impossible in either case.Hardware channel scanningContinuous channel scanning is implemented in hardware using channel-gain arrays (CGA).These programmable memory buffers contain a list of the channels and the gain settingsrequired for each input channel to be sampled. When the A/D board begins sampling, inputchannels are sampled in the sequence loaded into the channel-gain array.The use of on-board channel-gain arrays (CGA) overcomes many of the limitations associatedwith channel scanning using software and has the following advantages:• The channel sequence information may be setup once and then sampling initiated(and repeated) with a single command. Once initiated, the sampling process iscontrolled by the A/D board’s hardware.• Arbitrary sample sequences may be defined.• Within the limitations on the size of the CGA, different sampling frequencies maybe specified for different channels.• The speed of software-transfer methods such as interrupt and polled I/O is greatlyincreased, in many cases doubled. This is due to the fact that delays caused by thehost computer transferring channel and gain information before each sample istaken, are avoided.• Very accurate timing is achievable since the board hardware is optimized tocontrol the individual sub-systems on the board.• Advanced transfer methods such as DMA and repeat instructions are possible.DMA transfer is controlled directly by the hardware on the A/D board and thehost computer. This is not a very flexible arrangement, since it does not allowintervention by software to change the channels being scanned once a DMAtransfer has been initiated. A/D boards, which are capable of DMA but do nothave channel-gain arrays may only perform DMA transfers from a single inputchannel, whose address and required gain is setup by software before the DMAtransfer is initiated. Where channel-gain arrays are implemented, the on-boardhardware will automatically change the address and gain settings during the DMAtransfer. Where repeat instructions are used to transfer information, usually froman on-board FIFO, the sampling of multiple channels must continue to beperformed in the background. On A/D boards, which do not have channel-gainarrays, repeat instruction transfers may only be performed on a single inputchannel whose address and required gain is setup by software before the repeatinstruction transfer is initiated.Practical applicationsSome of the practical applications, which utilize the flexibility in the selection and throughputof individual channels, using hardware channel scanning, are detailed below.Sampling different channels at different frequenciesWhen signals with different frequencies are sampled (for example, a heart rateelectrocardiogram (ECG) with 300 beats/min and an electroencephalogram (EEG) with afrequency of 5 kHz), it is much more memory-efficient to sample each channel at around its