I'm experiencing the occasional flipped bit, always bit 4 or 5 when my serial port is under load. I'm using a FIFO on both recieve and transmitt. If I run in loopback (from the RX fifo straight to the Tx Fifo) I flip bits occasionaly. I wrote a simple test (INC ACC, Send & Loop) to see if it was the TX portion failing - No Problems. Then I wrote a RX test (Rx a byte, compare to Previous + 1, Toggle Port Pin if <> & loop) and sent it a value of 0 - 255 from a basic program. no Problems. As soon as I loop back the ports, I start flipping bits again. I'm begining to be suspicious that the TX & RX portions of the serial port are interfering with each other. My FIFO is clean. I've tried 3 different processors, and 2 differnet PSD's I'm running at 22.118Mhz (24Mhz part) my Serial port is currently set up for 9600 BPS. I've tried 2400, but it didn't help. I'm using MAX232's for my RS232 Drivers, Scope'd output shows a waveform + / - 8 Volts. Bit times are good @ 104us. Any ideas ?

(incorrect bytes are approximately 1 in 1000)