This page is intended to show the outline of the chip and to provide additional information that might not be clear from the data sheet.
The M1284 seems to have an internal problem where large amounts of serial data can choke the processor.
A capacitor of 100pF on the RX pin to ground can solve this problem.
More info : http://www.mcselec.com/index2.php?option=com_forum&Itemid=59&page=viewtopic&p=60860#60860