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Claudio
Documentation about RS485 serial communications
Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
---
Documentation/serial/00-INDEX | 2 +
Documentation/serial/serial-rs485.txt | 126 +++++++++++++++++++++++++++++++++
2 files changed, 128 insertions(+), 0 deletions(-)
create mode 100644 Documentation/serial/serial-rs485.txt
diff --git a/Documentation/serial/00-INDEX b/Documentation/serial/00-INDEX
index 07dcdb0..e09468a 100644
--- a/Documentation/serial/00-INDEX
+++ b/Documentation/serial/00-INDEX
@@ -14,6 +14,8 @@ riscom8.txt
- notes on using the RISCom/8 multi-port serial driver.
rocket.txt
- info on the Comtrol RocketPort multiport serial driver.
+serial-rs485.txt
+ - info about RS485 structures and support in the kernel.
specialix.txt
- info on hardware/driver for specialix IO8+ multiport serial card.
stallion.txt
diff --git a/Documentation/serial/serial-rs485.txt b/Documentation/serial/serial-rs485.txt
new file mode 100644
index 0000000..82ebc86
--- /dev/null
+++ b/Documentation/serial/serial-rs485.txt
@@ -0,0 +1,126 @@
+ RS485 SERIAL COMMUNICATIONS
+
+1. INTRODUCTION
+
+ EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the
+ electrical characteristics of drivers and receivers for use in balanced
+ digital multipoint systems.
+ This standard is widely used for communications in industrial automation
+ because it can be used effectively over long distances and in electrically
+ noisy environments.
+
+2. HARDWARE-RELATED CONSIDERATIONS
+
+ Some CPUs (e.g., Atmel AT91) contain a transceiver capable of working both
+ as RS232 and RS485. For these microcontrollers, the Linux driver should be
+ made capable of working in both modes, and proper ioctls (see later) should
+ be made available at user-level to allow switching from one mode to the
+ other, and vice versa.
+
+ On some other CPUs (e.g., Freescale imx25) the RS485 transceiver is not
+ integrated inside the microcontroller itself. Therefore, manufacturers who
+ use these microcontrollers to produce embedded boards need to connect an
+ external transceiver to some pin of the CPU.
+ On these architectures, therefore, no assumptions can be done at the
+ CPU-level about the presence of a RS485 transceiver, because the connection
+ (if any) is done outside the microcontroller. Moreover, even in case of
+ RS485 transceiver, the manufacturer is free to choose the CPU pin used for
+ the connection.
+
+
+3. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL
+
+ The Linux kernel provides the serial_rs485 structure (see [1]) to handle
+ RS485 communications. This data structure is used to set and configure RS485
+ parameters in the platform data and in ioctls.
+
+ Any driver for devices capable of working both as RS232 and RS485 should
+ provide at least the following ioctls:
+
+ - TIOCSRS485 (typically associated with number 0x542F). This ioctl is used
+ to enable/disable RS485 mode from user-space
+
+ - TIOCGRS485 (typically associated with number 0x542E). This ioctl is used
+ to get RS485 mode from kernel-space (i.e., driver) to user-space.
+
+ In other words, the serial driver should contain a code similar to the next
+ one:
+
+ static struct uart_ops atmel_pops = {
+ /* ... */
+ .ioctl = handle_ioctl,
+ };
+
+ static int handle_ioctl(struct uart_port *port,
+ unsigned int cmd,
+ unsigned long arg)
+ {
+ struct serial_rs485 rs485conf;
+
+ switch (cmd) {
+ case TIOCSRS485:
+ if (copy_from_user(&rs485conf,
+ (struct serial_rs485 *) arg,
+ sizeof(rs485conf)))
+ return -EFAULT;
+
+ /* ... */
+ break;
+
+ case TIOCGRS485:
+ if (copy_to_user((struct serial_rs485 *) arg,
+ ...,
+ sizeof(rs485conf)))
+ return -EFAULT;
+ /* ... */
+ break;
+
+ /* ... */
+ }
+ }
+
+
+4. USAGE FROM USER-LEVEL
+
+ From user-level, RS485 configuration can be get/set using the previous
+ ioctls. For instance, to set RS485 you can use the following code:
+
+ #include <linux/serial.h>
+
+ /* Driver-specific ioctls: */
+ #define TIOCGRS485 0x542E
+ #define TIOCSRS485 0x542F
+
+ /* Open your specific device (e.g., /dev/mydevice): */
+ int fd = open ("/dev/mydevice", O_RDWR);
+ if (fd < 0) {
+ /* Error handling. See errno. */
+ }
+
+ struct serial_rs485 rs485conf;
+
+ /* Set RS485 mode: */
+ rs485conf.flags |= SER_RS485_ENABLED;
+
+ /* Set rts delay before send, if needed: */
+ rs485conf.flags |= SER_RS485_RTS_BEFORE_SEND;
+ rs485conf.delay_rts_before_send = ...;
+
+ /* Set rts delay after send, if needed: */
+ rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
+ rs485conf.delay_rts_after_send = ...;
+
+ if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
+ /* Error handling. See errno. */
+ }
+
+ /* Use read() and write() syscalls here... */
+
+ /* Close the device when finished: */
+ if (close (fd) < 0) {
+ /* Error handling. See errno. */
+ }
+
+5. REFERENCES
+
+ [1] include/linux/serial.h
--
