RS 485 reception problem

[uttarawakhke]

Hi,
I am using rs485 between PIC24(master) and pic18(slave).
I am not getting correct data from slave. 15 bytes data is expected first byte and last 2 bytes are missing while remaining data that is received is correct.
Tried doing almost everything.. please help.

Thanks and regards,
Uttara

[Ttelmah]

On the last two bytes being missing, you are almost certainly turning off the transceiver, or turning it round (depending how it is wired) too early.

Remember that when a putc/printf returns there are _still_ two bytes in the hardware buffer. If you turn the transceiver off at this point, these will be lost....

Various solutions. Just delay for two byte times (simplest), or test the UART's TRMT bit, and wait for this to be set.

First byte being missing, could be multiple things. Is it missing, or are you getting garbage?. Missing, then look at whether the receiving device is enabling it's buffer quickly enough. Remember that the bus, and the lines to devices should all be wired to give the bus idle state when not driven. Otherwise you can see 'false' start bits that could lose part of the first byte. Consider a fractional delay between enabling the buffer, and sending.

Best Wishes

[Mike Walne]

[uttarawakhke]

Thanks Ttelmah.

I have given enough delay of 1sec before sending the data from master and also i check for TMRT == 1. still i am not ablr to receive last two bytes. now i am receiving first byte as garbage value.

Actually i am modifying RS232 compatible bootloader code to rs485. and full bootloader stack provided by microchip is in assembly. so can u please tell me that i have put rs485 control bit instructions at right place?

Please help.

[Mike Walne]

From here I can't tell if you've got a TX, RX or a TX and RX problem.
Judicious testing with a 'scope will give you some clues.

Mike

OR maybe convert TTL levels to RS232 and feed to a PC.