Modbus Master Communication - IDEC SmartAXIS FT9Z-1A01 User Manual

11: M C
ODBUS OM M UN I CATI ON

Modbus Master Communication

When configured as a Modbus master, the SmartAXIS sends communication requests to Modbus slaves to read/write data. Each
communication request is sent to a Modbus slave according to the configured request table.
Modbus master communication settings and request tables for Modbus slave stations can be programmed using the WindLDR
Function Area Settings. Communication with slave stations are performed in synchronism with user program execution, and the
communication data are processed at the END processing in the order of request numbers specified in the request table. When
request execution devices are designated, requests are executed only when the corresponding request execution device is turned
on. When request execution devices are not designated, all requests are executed continuously.
Modbus Master Communication Specifications
Item
Baud Rate (bps)
Data Bits
Stop bits
Parity
Slave Number
Maximum Number of Slaves
*2
Receive Timeout
Timeout between Characters
Transmission Wait Time
Retry Cycles
*1: A communication request becomes the broadcast when slave number 0 is specified. The broadcast communication request is received by all
Modbus RTU slaves. Modbus RTU slave does not reply to the broadcast communication. Broadcast can be used to write the same data to all
Modbus RTU slaves.
*2: Specifies the period of time before receiving a response frame from a slave.
Modbus Master Communication Start and Stop
When request execution devices are designated in the Modbus master request table, internal relays or data register bits as many
as the request quantity are allocated to execute Modbus master communication. The internal relays or data register bits are
allocated in the order of requests. For example, when internal relay M0 is designated as the request execution device, M0 is
allocated to request No. 1, M1 to request No. 2, and so on. To execute a request, turn on the corresponding request execution
device.
When communication is completed, the request execution device turns off automatically. When it is required to send requests
continuously, keep the corresponding request execution device on using a SET or OUT instruction.
When request execution devices are not designated, all requests programmed in the request table are executed continuously.
Communication Completion and Communication Error
Modbus communication finishes when a read or write process is completed successfully or when a communication error occurs.
Communication error occurs when communication failure has repeated more than the designated retry cycles or when the master
station does not receive response within the designated receive timeout period. When a communication error occurs, the request
is canceled and the next request is processed. When the error status data register is designated, the communication status of each
request can be confirmed.
Note: Modbus master processes a maximum of one Modbus request per scan.
11-2
9600, 19200, 38400, 57600, 115200
8 bits (fixed)
1, 2 bits
Even, Odd, None
1 to 247 (0: broadcast slave number)
31
10 to 2550 ms (in increments of 10 ms)
10 ms
1 to 5000 ms (in increments of 1 ms)
1 to 10
S A XI S P / L U '
M AR T R O I TE SER S
Description
*1
M FT9Y-B 1378
ANUAL