c - automatically changing RTS for RS-485 communication

Question

I'm trying to setup half duplex communication in my program. I have my RS485 transceiver using the RTS flag (TIOCM_RTS) to toggle back and forth between transmit and receive. And to send/receive data I need to change RTS flag manually:

1. Set RTS to High.

2. Send data.

3. Set RTS to low.

   int setRTS(int level) {
       int status;
       ioctl(ser_port, TIOCMGET, &status);
       if(level) {
           status |= TIOCM_RTS;
       } else {
           status &= ~TIOCM_RTS;
       }
       ioctl(ser_port, TIOCMSET, &status);
       return 1;
   }
   

My question is: shouldn't the linux kernel be able to switch RTS automatically? And how to ensure that data was sent before calling setRTS(0)?

Answer 1

shouldn't the linux kernel be able to switch RTS automatically?

Yes, there is kernel framework for this starting in Linux 3.0.
There are two ioctls in include/uapi/asm-generic/ioctls.h:

#define TIOCGRS485      0x542E
#define TIOCSRS485      0x542F

to retrieve and configure a tty serial port driver in RS-485 mode.
These ioctls use the struct serial_rs485:

 /*
  * Serial interface for controlling RS485 settings on chips with suitable
  * support. Set with TIOCSRS485 and get with TIOCGRS485 if supported by your
  * platform. The set function returns the new state, with any unsupported bits
  * reverted appropriately.
  */

 struct serial_rs485 {
         __u32   flags;                  /* RS485 feature flags */
 #define SER_RS485_ENABLED               (1 << 0)        /* If enabled */
 #define SER_RS485_RTS_ON_SEND           (1 << 1)        /* Logical level for
                                                            RTS pin when
                                                            sending */
 #define SER_RS485_RTS_AFTER_SEND        (1 << 2)        /* Logical level for
                                                            RTS pin after sent*/
 #define SER_RS485_RX_DURING_TX          (1 << 4)
         __u32   delay_rts_before_send;  /* Delay before send (milliseconds) */
         __u32   delay_rts_after_send;   /* Delay after send (milliseconds) */
         __u32   padding[5];             /* Memory is cheap, new structs
                                            are a royal PITA .. */
 };

I've used this RS-485 capabilty on Atmel and Etrax SoCs, but otherwise implementation of these ioctls in Linux UART/USART drivers is very sparse.
If your driver doesn't have it, then consider implementing it yourself. You could use the implementation in drivers/tty/serial/atmel_serial.c as a guide. Also read the Linux kernel document for RS485.