HPLC System Control and Automation

The A/D board can reside either in the detector, an interface box, or in the computer. If the board is in the computer, it will have analog input terminals similar to a strip chart recorder or it will be connected to an interface box that will have this type of connector. If the A/D card is in the detector, the detector will have some type of digital interface port and a cable to connect it to the computer. In the personal computer world, once the signal is digitized outside the computer, it will be sent to the computer over one of three types of communication cables parallel, serial, or GPIB. 

The GPIB or IEE-488 cable is a bidirectional serial cable carrying module addressing information lines as well as signal and housekeeping lines. It can be used to send and collect information from different instruments on a cable bus. It is often used in instrumentation to automate a series of discrete instrument modules connected through a system controller. 

HPLC systems operate on a master-slave arrangement in setting up for automation. One module sets the timing and initiates the process, and the remaining modules accept a signal and follow the leader. In a gradient HPLC system, the master module can be a microprocessor-based controller, a computer software-based controller, an autosampler, or an integrator. 

A system controller sends out pump speed control signals to set flow rates and to generate solvent gradients. It should also be able to send 0- to 5-mV contact closure signals to actuate external switches and equipment. It also may be able to send out a 0- to 30-V powered contact closure signal needed to run solenoid valves used to control air pressure and liquid flows. Finally, the 

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