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Problems with serial driver for RS485 applications on TWRK60 , TWR-SER and MQX3.7

Question asked by Luca Gotti on May 13, 2011
Latest reply on Feb 19, 2014 by Ian Bentall


I'm developing an application with MQX3.7 compiled with IAR compiler (v. 6.1) using the TWRK60 toolkit as a protoype board .

In order to us the RS422 / RS485 transceiver ( U5 on the TWR-SER ) as a RS485 driver, i've to set the following jumpers : J17, J18 and J19 in pos 2-3

and i've installed three jumpers on the J13 header on pos 1-2 , 3-4 and 5-6.


I tried to use the IO_SERIAL_HW_485_FLOW_CONTROL option in opening my serial port as follows :


        /* HW 485 flow control on chip*/
        rs485_dev = fopen( RS485_CHANNEL, (char const *)(IO_SERIAL_HW_485_FLOW_CONTROL ) );

When i try to send a message on the rs485 device i see the RTS signal being driven sincronous with the char ( just before the fisrt character and just after the last char ) but the problem is in the polarity of the signal : instead of being driven HIGH during data transmission, it is driven LOW ( wich means enabling only the receiver of the transceiver ).

Is this a bug of MQX or there is a logical reason for MQX to drives this pin LOW instead of HIGH ..??

I've solved the problem simply opening the RS485 channel without the IO_SERIAL_HW_485_FLOW_CONTROL option and adding the hanshake of the RTS pin directly by the lwgpio driver .


However i  think that this behaviour it would be corrected in MQX if there is any valid reason ....