works, but license issues due to bcm2835 lib

pyserialext/Makefile

@@ -0,0 +1,13 @@
+CFLAGS=-I/opt/bcm2835/include
+
+writec.so:	writec.o /opt/bcm2835/lib/libbcm2835.a
+	$(LD) -shared -o $@ $^
+
+writec.o:	writec.c
+
+.c.o:
+	$(CC) $(CFLAGS) -c $<
+
+.PHONY: clean
+clean:
+	-rm -f *.so *.o

pyserialext/RS485Ext.py

@@ -0,0 +1,19 @@
+import serial.rs485
+import serial.serialutil
+import ctypes
+
+
+class RS485Ext(serial.rs485.RS485):
+    def __init__(self, *args, **kwargs):
+        super(RS485Ext, self).__init__(*args, **kwargs)
+        self.writec = ctypes.cdll.LoadLibrary('writec.so')
+        fd = self.fileno()
+        r = self.writec.set_rs485_mode(fd)
+
+    def write(self, b):
+        d = serial.serialutil.to_bytes(b)
+        l = len(d)
+        fd = self.fileno()
+        r = self.writec.writec(fd, d, l)
+        return r
+

pyserialext/writec.c

@@ -0,0 +1,31 @@
+#include <unistd.h>
+#include <termios.h>
+#include <sys/ioctl.h>
+#include <stdio.h>
+#include <linux/serial.h>
+#include <errno.h>
+#include <string.h>
+#include <stdint.h>
+#include <bcm2835.h>
+
+
+const uint8_t DE_PIN = RPI_V2_GPIO_P1_11;
+
+int set_rs485_mode(int fd) {
+  bcm2835_init();
+  bcm2835_gpio_fsel(DE_PIN, BCM2835_GPIO_FSEL_OUTP);
+  bcm2835_gpio_write(DE_PIN, LOW);
+}
+
+ssize_t writec(int fd, char *buf, size_t count) {
+  bcm2835_gpio_write(DE_PIN, LOW);
+  ssize_t r = write(fd, buf, count);
+  uint8_t lsr;
+  do {
+    int r = ioctl(fd, TIOCSERGETLSR, &lsr);
+  } while (!(lsr & TIOCSER_TEMT));
+  bcm2835_gpio_write(DE_PIN, HIGH);
+
+  return r;
+}
+

snippets/test1.py

@@ -1,5 +1,6 @@
 import serial.rs485
 ser=serial.rs485.RS485(port='/dev/ttyAMA0',baudrate=2400)
 ser.rs485_mode = serial.rs485.RS485Settings(False, True)
-ser.write('a test'.encode('utf-8'))
+while True:
+    ser.write('a test'.encode('utf-8'))
 

snippets/test5.py

@@ -0,0 +1,18 @@
+from pymodbus.client.sync import ModbusSerialClient
+import serial.rs485
+import RS485Ext
+
+#ser=serial.rs485.RS485(port='/dev/ttyAMA0',baudrate=1200)
+#ser.rs485_mode = serial.rs485.RS485Settings(rts_level_for_tx=False, 
+#                                            rts_level_for_rx=True,
+#                                            delay_before_tx=0.005,
+#                                            delay_before_rx=-0.0)
+ser=RS485Ext.RS485Ext(port='/dev/ttyAMA0', baudrate=1200)
+
+client = ModbusSerialClient(method='rtu')
+client.socket = ser
+client.connect()
+result = client.read_holding_registers(address=0x2000, count=2, unit=1)
+print(result)
+print(result.registers)
+client.close()