Modbus Rtu Master Communication - IDEC FC6A-C16K1CE Communications Manual

6: M C
ODBUS OMMUNICATION

Modbus RTU Master Communication

When configured as a Modbus RTU master, the FC6A Series MicroSmart sends communication requests to Modbus RTU slaves to
read/write data. Each communication request is sent to a Modbus RTU slave according to the configured request table.
Modbus RTU master communication settings and request tables for Modbus RTU 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 RTU Master Communication Specifications
Baud Rate (bps)
Data Bits
Stop bits
Parity
Slave Number
Maximum Number of Slaves
Maximum Cable Length
*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 RTU Master Communication Start and Stop
When request execution devices are designated in the Modbus RTU master request table, internal relays or data register bits as
many as the request quantity are allocated to execute Modbus RTU 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 RTU 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.
6-2
Item
FC6A S M S A
ERIES ICRO MART
9,600, 19,200, 38,400, 57,600, 115,200
8 bits (fixed)
1, 2 bits
Even, Odd, None
1 to 247 (0: broadcast slave number)
RS-232C: 1
RS-485: 31
RS-232C: 5 m
RS-485: 200 m
10 to 2,550 ms (in increments of 10 ms)
10 ms
0 to 5,000 ms (in increments of 1 ms)
1 to 10
- -O T C
LL IN NE YPE OMMUNICATION
Description
*1
M FC9Y-B1730
ANUAL