Feature: class ModbusRTUDelay allows for calculation of minimum inter-frame breaks.

examples/RTU/ModbusRTUClientParameters/ModbusRTUClientParameters.ino

@@ -16,24 +16,12 @@
 
 constexpr auto baudrate { 19200 };
 
-// Calculate preDelay and postDelay in microseconds as per Modbus RTU Specification
-//
-// MODBUS over serial line specification and implementation guide V1.02
-// Paragraph 2.5.1.1 MODBUS Message RTU Framing
-// https://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf
-constexpr auto bitduration { 1.f / baudrate };
-constexpr auto wordlen { 9.6f }; // try also with 10.0f
-constexpr auto preDelayBR { bitduration * wordlen * 3.5f * 1e6 };
-constexpr auto postDelayBR { bitduration * wordlen * 3.5f * 1e6 };
-
 void setup() {
   Serial.begin(9600);
   while (!Serial);
 
   Serial.println("Modbus RTU Client Toggle w/ Parameters");
 
-  RS485.setDelays(preDelayBR, postDelayBR);
-
   // start the Modbus RTU client in 8E1 mode
   if (!ModbusRTUClient.begin(baudrate, SERIAL_8E1)) {
     Serial.println("Failed to start Modbus RTU Client!");

keywords.txt

@@ -7,6 +7,7 @@
 #######################################
 
 ArduinoModbus	KEYWORD1
+ModbusRTUDelay	KEYWORD1
 ModbusRTUClient	KEYWORD1
 ModbusRTUServer	KEYWORD1
 ModbusRTUClient	KEYWORD1
@@ -21,6 +22,9 @@ poll	KEYWORD2
 end	KEYWORD2
 setTimeout	KEYWORD2
 
+preDelay	KEYWORD2
+postDelay	KEYWORD2
+
 beginTransmission	KEYWORD2
 write	KEYWORD2
 endTransmission	KEYWORD2

src/ModbusRTUClient.cpp

@@ -26,6 +26,8 @@ extern "C" {
 
 #include "ModbusRTUClient.h"
 
+#include "ModbusRTUDelay.h"
+
 ModbusRTUClientClass::ModbusRTUClientClass() :
   ModbusClient(1000)
 {
@@ -42,6 +44,8 @@ ModbusRTUClientClass::~ModbusRTUClientClass()
 
 int ModbusRTUClientClass::begin(unsigned long baudrate, uint16_t config)
 {
+  _rs485->setDelays(ModbusRTUDelay::preDelay(baudrate), ModbusRTUDelay::postDelay(baudrate));
+
   modbus_t* mb = modbus_new_rtu(_rs485, baudrate, config);
 
   if (!ModbusClient::begin(mb, 0x00)) {

src/ModbusRTUDelay.h

@@ -0,0 +1,50 @@
+/*
+  This file is part of the ArduinoModbus library.
+  Copyright (c) 2018 Arduino SA. All rights reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#ifndef _MODBUS_RTU_DELAY_H_INCLUDED
+#define _MODBUS_RTU_DELAY_H_INCLUDED
+
+class ModbusRTUDelay
+{
+public:
+  /* Do not allow construction or copying. */
+  ModbusRTUDelay() = delete;
+  ModbusRTUDelay(ModbusRTUDelay const &) = delete;
+
+  /* Calculate __minimum__ preDelay and postDelay in
+   * microseconds as per Modbus RTU Specification.
+   *
+   * MODBUS over serial line specification and implementation guide V1.02
+   * Paragraph 2.5.1.1 MODBUS Message RTU Framing
+   * https://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf
+   */
+  static unsigned long const preDelay(unsigned long const baudrate)
+  {
+    double const bit_duration = 1. / baudrate;
+    double const word_len = 9.6f; // try also with 10.0f
+    double const pre_delay_us = bit_duration * word_len * 3.5f * 1e6;
+    return static_cast<unsigned long>(pre_delay_us);
+  }
+  static unsigned long const postDelay(unsigned long const baudrate)
+  {
+    return preDelay(baudrate);
+  }
+};
+
+#endif /* _MODBUS_RTU_DELAY_H_INCLUDED */

src/ModbusRTUServer.cpp

@@ -26,6 +26,9 @@ extern "C" {
 
 #include "ModbusRTUServer.h"
 
+#include "ModbusRTUDelay.h"
+
+
 ModbusRTUServerClass::ModbusRTUServerClass()
 {
 }
@@ -40,6 +43,8 @@ ModbusRTUServerClass::~ModbusRTUServerClass()
 
 int ModbusRTUServerClass::begin(int id, unsigned long baudrate, uint16_t config)
 {
+  _rs485->setDelays(ModbusRTUDelay::preDelay(baudrate), ModbusRTUDelay::postDelay(baudrate));
+
   modbus_t* mb = modbus_new_rtu(_rs485, baudrate, config);
 
   if (!ModbusServer::begin(mb, id)) {