The TM503 is a three-wide power module compatible with all TM500-Series plug-ins. Up to three single width modules can be plugged into the TM503, or one single and one double width module can be accommodated. The TM500-Series of plug-ins includes oscilloscopes, function generators, scope calibration equipment, DMM's, counters and power supplies. Also choose from pulse generators, operational amplifiers, bandpass filters, and current probe amplifiers. You may be able to fit every instrument you'll need into one small mainframe. Plug-ins sold separately.