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I am developing an application on a BeagleBoneBlack. Embedded linux is quite new for me =/

The problem I am facing is that I want to implement a RS485 communication, I wrote a piece of C++-Code by using the SeriaLib-Library, for the moment a dedicated GPIO is used to toggle the direction of the RS485 interface. All has been fine till this point.

To write a message the function write is used a below

write(fd,Message,Lenght);

The only problem is to toggle the DIRECTION-Pin immediately after the last byte is written on the BUS =(

While developing on microcontrollers its no problem to react on various interrupts and so on.

Is there any solution - deep in the kernel? - to toggle a pin DIRECT after the succesful transmit of the message?

Marriott81
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Caleb
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    Well it's hardly "embedded" - you have a full desktop OS on there! It's just _different_ hardware than you're used to. – Lightness Races in Orbit Jan 15 '14 at 16:01
  • @LightnessRacesinOrbit - you really have no idea what installation is on there. Uncounted tiny, clearly "embedded" devices have linux *kernels* these days (pocket routers, SD cards that do wifi upload, etc), being a "full desktop OS" or not depends on the *unspecified userspace*. This problem really comes down to the use of a multitasking operating system, on hardware that may or may not support a transmission complete interrupt. – Chris Stratton Jan 15 '14 at 16:32

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Normally on a small microcontroller upon fifoing the last character you can either enable a transmission complete interrupt, or just busy wait on the transmit complete flag in your send routine, then disable the driver.

However, some heavier-weight MCU's don't have a transmission complete interrupt, and a multitasking OS may dislike busy waiting (and introduce undesired latency if you yield between polls). Two ways of dealing with that come to mind.

  1. Since you have a driver to disable, it sounds like you have a single bidirectional pair, and will receive everything you transmit. Coupled with some state tracking, you could then use the receive interrupt to know that your last character has been sent, and disable the driver. (Also handle RX errors if somebody else jams the bus)

  2. Determine the baudrate-dependent time from enqueueing the last character until transmission complete, and set a timer interrupt for that amount of time, then disable the driver in the ISR.

Chris Stratton
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  • Hello Chris, you are right with the single bidirectional signal data line. It is a 2-wire RS485. To your suggested points (1) and (2). (1) Could be a solution, but i dont want spent too much intelligence into the transmit algorithm. (2) This is exactly the way i am doing right now - this can't a proper solution. My further thoughts are: - Usage of the UART1-RTS to enable the transceiver. I guess this should have the same effect like a DIRECTION-PIN. --> Is there a method to the RTS-state in my application? I really have to know the time of a complete message transmission. – Caleb Jan 16 '14 at 10:25
  • I suggest you spend the time to work out the state logic, or if that is too much build a hardware driver timeout. – Chris Stratton Jan 16 '14 at 11:18