I made RS232/RS485 convertor. I've some problem that sometime SN75176 not working properly: pin R of SN75176 goes to low that's reqiesting serial interrupt of PC (via MAX232). SN75176 is connected biasing resistor 5k (A,B pins).
What is wrong in my design? help me!

PC max232 SN75176
RXD---|7 10|------|R |
TXD---|8 9|------|D B|(5k to GND)
DTR---| |----+-|RE A|(5k to VCC)
'-|DE |
GND----to all GND