Telecommunications equipment maker Marconi said Thursday that it will sell its data systems subsidiary to Danaher for $400 million. The London-based company expects the deal to close by March 31, assuming it clears regulatory requirements. The unit, based in Wood Dale, Ill., makes digital jet printers, graphics, and laser printing systems and provides parts support services. Marconi said the data division generated revenue of $344 million during the 12 months ended March 31 of last year. Danaher makes electronics test equipment as well as environmental monitoring gear and hand tools.