FreeMODBUS - A Modbus ASCII/RTU and TCP implementation

The hardware requirements are minimal - Any reasonable microcontroller with a serial interface, at least some RAM to hold the modbus frames should suffice.

• A UART which support and interrupts and has at least an receiver buffer full and transmit buffer empty interrupt.
• A timer capable of creating the t3.5 character timeout timer for Modbus RTU.

For the software part a simple event queue is necessary. The STR71X/FreeRTOS port uses FreeRTOS queues for this purpose to reduce the time spent in the Modbus task. Smaller microcontrollers will most likely come without an operating system. In that case a simple implementation with a global variable is possible (The Atmel AVR port uses such an implementation).

The actual memory requirements depend on the used modules. The first table shows the required memory with all supported functions compiled in. Values for the ARM where obtained using the GNUARM compiler collection 3.4.4 with -O1 in ARM mode. The AVR values where obtained using the WinAVR compiler collection 3.4.5 with -Os.

Module	ARM Code	ARM RAM (static)	AVR Code	AVR RAM (static)
Modbus RTU (Required)	1132Byte	272Byte	1456Byte	266Byte
Modbus ASCII (Optional)	1612Byte	28Byte	1222Byte	16Byte
Modbus Functions [1]	1180Byte	34Byte	1602Byte	34Byte
Modbus Core (Required)	924Byte	180Byte	608Byte	75Byte
Porting Layer (Required [2])	1756Byte	16Byte	704Byte	7Byte
Totals	7304Byte	530Byte	5592Byte	398Byte

[1]: Actual size depends on the available Modbus functions. They are configurable in the header file mbconfig.h.
[2]: Depends on the hardware.