diff --git a/src/modbus-rtu.c b/src/modbus-rtu.c
index f2911d6..190298e 100644
--- a/src/modbus-rtu.c
+++ b/src/modbus-rtu.c
@@ -916,7 +916,7 @@ int modbus_rtu_set_serial_mode(modbus_t *ctx, int mode)
                 return -1;
             }
             // Set
-            rs485conf.flags = SER_RS485_ENABLED;
+            rs485conf.flags |= SER_RS485_ENABLED;
             if (ioctl(ctx->s, TIOCSRS485, &rs485conf) < 0) {
                 return -1;
             }