MODBUS Slave Protocol

Smart LCD MODBUS Salve Product can be accessed by MODBUS Master devices for real-time control and access.

Basic

Modbus Communication

The Modbus is a protocol, which is used to communicate between a host and one or multiple slave devices. Over the Modbus the host is able to communicate with up to 254 slaves. It can read or write data areas on these devices. The SMART LCD is operated as such a slave device. There are mainly two modes of communication: The binary RTU Mode and the ASCII Mode. The SMART LCD is operating in RTU Mode. Therefore, we only describe the RTU mode within this document.

The host issues a package, which consists of:

• An 8 Bit Device Address, to indicate which slave device shall be addressed
• An 8 Bit Function Code, to tell the addressed slave which data area is read or written
• Specific data, like addresses and data values
• A 2-Byte Error Check CRC to determine, whether the previous data are correct

< - - - - - - - - - - - - - - - - - - - - MODBUS Package - - - - - - - - - - - - - - - - - - - - >
Device Address (1byte)	Function (1byte)	Data (252bytes(max.))	Error Check (2bytes)
	< - - - - - - - - - - MODBUS Frame - - - - - - - - - - >

When the host receives the previous frame reply from the slave device, the host needs an interval of 5ms before it can continue to send the next frame command. Since the SMART LCD is a Modbus slave device, a device address must be assigned to it. This address will be defined while designing the GUI with TOPWAY’s SGTools.

Modbus Function Codes

Function Code	Function Definition	Address Range	Object Type
0x03	Read Multiple Holding Registers	0x0000 ~ 0xFFFF	2 Bytes
0x06	Write Single Holding Register	0x0000 ~ 0xFFFF	2 Bytes
0x10	Write Multiple Holding Registers	0x0000 ~ 0xFFFF	2 Bytes
0x11	Report Slave ID
0x14	Read File Record	1 File with 2048 Records of 128 Byte

To access memory locations in the SMART LCD the host has to issue a Read or Write function, which includes a specific address. Each Read or Write function accesses a 2-Byte location within an area of up to 64k words. Since the multiple data areas (N16, N32, N64, etc.) are together larger than 64k Words, the different memory areas are separated into pages, a total of 5 pages. To select a specific page the Modbus Page Map Register must be written with the page number. The Page Map Register is located within the set of Special Function Registers. The Special Function Registers (SFR) are accessible on all pages at the same location. They are placed within the final 4k words of all pages.

Address Mapping

The following table gives an overview over the relationship between Modbus addresses, Modbus Memory Pages and the SMART LCD Address space

VP variable addressing

In SGTools, MENU --> View --> Show Modbus Holding Register Address

can show up the VPs corresponding MODBUS ADDRESS for the display UI.

MODBUS HOST can access those VP in MODBUS address accordantly, in real time.

MODBUS address mapping

VP type	VP address	MODBUS PAGE	MODBUS Page:Address
VP_STR	0x000000 0x000080 : 0x01DF80	0
				reserved
VP_REG	*1
VP_N16	0x080000...	4	0x0000 - 0x7FFF
			0x8000 - 0xEFFF
			0xF000 - 0xFFFF
VP_N32	0x020000...	1	1:0x0000....
VP_N64	0x030000...	6	6:0x0000....
VP_G16	0x060000...	3	3:0x0000....
VP_BMP	0x040000...	2	20x0000...
VP_REG	0xFFFF00....		---

VP type	SMART LCD VP Address	Modbus PAGE: Address
VP_STR	0x000000, 0x000080, - 0x01DF80	0:0x0000, 0:0x0040 - 0:0xEFC0
VP_REG	Special Function Registers	0:0xF000, 0:0xF001 - 0:0xFFFF
VP_N32	0x020000 0x020004 : 0x02FFBC	1:0x0000 1:0x0002 : 1:0x7FDE
VP_REG	0x02FFE0 Timer 0 Count Reg. 0x02FFE4 Timer 1 Count Reg. : 0x02FFFC Timer 7 Count Reg.	1:0x7FF0 1:0x7FF2 : 1:0x7FFE
VP_N64	0x030000 0x030008 : 0x03DFF8	6:0x8000 6:0x8004 : 6:0x7FFC
VP_REG	Special Function Registers	6:0xF000 : 6:0xFFFF
VP_N16	0x080000 0x080002 : 0x08FFFE	4:0x0000 4:0x0001 : 4:0x7FFF
Reserved	Reserved	4:0x8000 : 4:0xEFFF
VP_REG	Special Function Registers	4:0xF000 : 4:0xFFFF
VP_BMP	0x040000 : 0x05DFFE	2:0x0000 : 2:0xEFFF
VP_REG	Special Function Registers	2:0xF000 : 2:0xFFFF
VP_G16	0x060000 : 0x07DFFE	3:0x0000 : 3:0xEFFF
VP_REG	Special Function Registers	3:0xF000 : 3:0xFFFF
VP_Flash (Flash Area)	120K Bytes Flash memory Non-volatile memory area, which will remain unchanged even after power down	5:0x0000 : 5:0xEFFF
VP_REG	Special Function Registers	5:0xF000 : 5:0xFFFF