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 standard protocol (with RTU or ASCII mode) 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 Slave Product works as a slave device in RTU mode. Therefore, we only describe the RTU mode within this document.

The MODBUS host communication frame consists:
- 1byte of Device Address, to indicate which slave device shall response
- 1byte of Function Code, to tell the addressed slave which function is going to do
- Data content for PAGE, Addresses and data values
- 2-Byte CRC for Error Checking

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

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 in TOPWAY’s SGTools.

MODBUS Function Codes

MODBUS 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

MODUBS Address Mapping

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. 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.

In SGTools, MENU --> View --> Show Modbus Holding Register Address can show up the VPs corresponding MODBUS ADDRESS for the display UI.

Relationship between MODBUS PAGE:addresses and the SMART LCD VPs

VP type	SMART LCD VP Address	MODBUS PAGE: Address	note
VP_STR	0x000000, 0x000080 - 0x01DF80	0:0x0000, 0:0x0040 - 0:0xEFC0
VP_N32	0x020000, 0x020004 - 0x02FFDC	1:0x0000, 1:0x0002 - 1:0x7FEE
TIMER COUNTER	0x02FFE0, 0x02FFE4 - 0x02FFFC	1:0x7FF0, 1:0x7FF2 - 1:0x7FFE	Timer 0, Timer 1 - Timer 7
VP_N64	0x030000, 0x030008 - 0x03FFF8	6:0x0000, 6:0x0004 - 6:0x7FFC
VP_N16	0x080000, 0x080002 - 0x08FFFE	4:0x0000 - 4:0x7FFF
VP_BMP	0x040000 - 0x05DFFE	2:0x0000 - 2:0xEFFF
VP_G16	0x060000 - 0x06EFFF	3:0x0000 - 3:0xEFFF
VP_Flash	120kB Flash memory	5:0x0000 - 5:0xEFFF	remain unchanged even after power down
VP_REG	0xFFF000 - 0xFFFFFF	--	VP_Reg can not be accessed directly via MODBUS
SFR	--	n:0xF000 - n:0xFFFF	SFR provide direct access of some VP_REG functions

MODBUS Function Codes Details

The SMART LCD supports five different MODBUS functions: 0x03, 0x06, 0x10, 0x11, 0x14

Function Code 0x03 - “Read Multiple Holding Registers”

This function code allows the host to read up to 250 Bytes from the memory (VP and SFR) of the SMART LCD at once.

Host to Smart LCD Frame

Byte Seq	Content	No. of Byte	Valid Range	note
0	MODBUS Device Address	1	0x01 … 0xF7
1	Function Code	1	0x03
2	Start Address to be read (*1)	2	0x0000 … 0xFFFF	Address location that MODBUS PAGE assigned via SFR
4	Quantity of data	2	0x0001 … 0x007D	no. of 16 Bit data values to be read
6	CRC	2	0x0000 … 0xFFFF

SMART LCD Response Frame

Byte Seq	Content	Byte Count	Valid Range	note
0	MODBUS Device Address	1	0x01 … 0xF7
1	Function Code	1	0x03
4	Byte Count N	1	0x02 … 0xFA	must be an even number, must be double of the requested "Quantity of data" in16 Bit values
6	N Bytes of data	N	0x00 … 0xFF
6 + N	CRC	2	0x0000 … 0xFFFF

Communication Example (Read a VP_N32 value form Smart LCD (device id = 01)

Seq	direction	MODBUS Frame	Descriptions
1	Host send	01 06 F0 01 00 01 2A CA	Set the Smart LCD MODBUS PAGE =1; omit this Frame for continue access the same PAGE
2	Smart LCD reply	01 06 F0 01 00 01 2A CA
3	Host send	01 03 00 00 00 02 C4 0B	Read multiple data form address 00 00, and get 00 02 byte
4	Smart LCD reply	01 03 04 33 44 11 22 39 2B	no. of byte =04; Data value = 0x11223344

Accessing the Smart LCD

To access data the host should do following steps using the Modbus protocol:

1. Use “Write Single Holding Register” with function code (0x06) to address 0xF001 and the data value of the desired memory page (0x01 … 0x05)
2. Write to or read from the desired address within the selected page with: “Read Multiple Holding Registers” with function code (0x03) “Write Single Holding Register” with function code (0x06) "Write Multiple Holding Registers” with function code (0x10)

Each Modbus address holds 16 Bit of data. Therefore, it might be necessary to read multiple address locations:

• STR, up to 64 consecutive accesses might be necessary to read an entire string, One single access will manipulate two characters.
• N16, just one access is necessary to access one N16 variable. Multiple N16 variables might be accessed with a single Modbus command.
• N32, two accesses are necessary to access one N32 variable. Multiple N32 variables might be accessed with a single Modbus command.
• N64, four accesses are necessary to access one N64 variable. Multiple N64 variables might be accessed with a single Modbus command.
• Bitmap, one access can manipulate 16 pixels of a bitmap variable. Multiple pixels might be accessed with a single Modbus command.
• Graph, one access can manipulate one data point of a graph. Multiple data points might be accessed with a single Modbus command.

“Read Multiple Holding Registers” with function code (0x03) allows accessing of up to 125 consecutive address locations at once, with each location holding two Bytes of data.

“Write Multiple Holding Registers” with function code (0x10) allows accessing of up to 123 consecutive address locations at once, with each location holding two Bytes of data.

Access to the Special Function Registers, which are mapped to all pages, should not exceed the required Byte count assigned to each SFR. In such a case the SMART LCD would reply the host with an error message.

Note: Since the Modbus protocol uses a “Big-Endian” representation of data, the first Byte of each 16 Bit word represents the upper 8 Bit of the data word. In contrary the memory organization of the SMART LCD is “Little-Endian” requiring the host to swap upper and lower Byte of each transaction.