Moreover, any hints or simple code for check the serial byte timeout?
i have no idea to do that.

would u mind give some hints to me.

would you mind not using stupid SMS.

one possible method:
slaves, which must listen all them time do not care about time out, if a message is incomplete, they just ignore it. Using the 9-bit protocol a slave will always know start of message. Since the slave will only respond to a complete request from the master, the slave will never tie the line except when the master does not receive correctly.

The master will ask a slave to respond and wait time x for the first byte. If the first byte is not received in thatv window, the master send a universal select several times to force an "all shut up" on the line. If the first byte is received in a timely fashion the master will time out if the next byte is not received in say 2 character times. When a timeout happens the master must spend a while sending "all shut up" to enbsure that a garbled slave transmission is completed to have the line free.

The above will work in a sytem that works, if you wire the system in such a way you get tons of noise you may have to take other much more complicated meaasures.

Make sure you use 485 drivers that can handle two drivers fighting during "all shut up"

Erik



Erik