diff --git a/configure.ac b/configure.ac
index 5b933d6..e01a7e3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -127,7 +127,7 @@ if test "x$HAVE_WINSOCK2_H" = "xyes"; then
    AC_SUBST(LIBS)
 fi
 
-# Check for RS485 support (recent Linux kernels)
+# Check for RS485 support (Linux kernel version 2.6.28+)
 AC_CHECK_DECLS([TIOCSRS485], [], [], [[#include <sys/ioctl.h>]])
 
 AC_CONFIG_FILES([
diff --git a/doc/modbus_rtu_get_serial_mode.txt b/doc/modbus_rtu_get_serial_mode.txt
index 58162e4..beab123 100644
--- a/doc/modbus_rtu_get_serial_mode.txt
+++ b/doc/modbus_rtu_get_serial_mode.txt
@@ -30,7 +30,7 @@ currently used by the libmodbus context:
  automation because it can be used effectively over long distances and in
  electrically noisy environments.
 
-This function is only available on recent Linux kernels.
+This function is only available on Linux kernels 2.6.28 onwards.
 
 
 RETURN VALUE
diff --git a/doc/modbus_rtu_set_serial_mode.txt b/doc/modbus_rtu_set_serial_mode.txt
index 077982a..0876d95 100644
--- a/doc/modbus_rtu_set_serial_mode.txt
+++ b/doc/modbus_rtu_set_serial_mode.txt
@@ -30,7 +30,7 @@ mode:
  automation because it can be used effectively over long distances and in
  electrically noisy environments.
 
-This function is only available on recent Linux kernels.
+This function is only available on Linux kernels 2.6.28 onwards.
 
 
 RETURN VALUE