IAI America ROBO Cylinder Series - Manual (Page 189)

6. Modbus
ASCII
181
Modbus
Caution
x The sizes of send/receive buffers are set to 256 bytes for the RC controllers, respectively. Make
sure to keep the messages small enough such that messages sent from the host side do not
exceed the receive buffer and data requests do not exceed send buffer.
x If the number of data items results in an odd number of bytes, caution must be taken for the reasons
below.
The data is communicated on a byte-by-byte basis in Modbus communication.
In many cases, however, the data is treated in units of 2 bytes on the master side. If the number of
data items becomes odd, 00
H
(i.e., NULL) may be added automatically at the end of a packet in some
cases.
RC controllers are configured such that the Modbus RTU is basically used as the interface on the
master side. Since the controller normally stands by for reception in the RTU mode, and then makes
judgment whether the code is ASCII or not after the reception, it cannot manage header/delimiter
fields. For this reason, communication in the ASCII mode is disabled in such cases.
Example: In case of querying output ports of axis No. 0
1 byte
1 word
Fig.6.2
NULL is added