Chapter 6. Serial I/O, SNP & RTU Protocols
226 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD
6.3 RTU Slave Protocol
RTU protocol is a query-response protocol used for communication between the RTU device and a host
computer, which is capable of communicating using RTU protocol. The host computer is the master
device and it transmits a query to a RTU slave, which responds to the master. The RTU slave device
cannot query; it can only respond to the master. A PACSystems CPU can only function as an RTU slave.
The RTU data transferred consists of 8-bit binary characters with an optional parity bit. No control
characters are added to the data block; however, an error check (Cyclic Redundancy Check) is included
as the final field of each query and response to ensure accurate transmission of data.
Note: You should avoid using station address 1 for any other Modbus slave in a PACSystems
control system because the default station address for the PACSystems CPU is 1. The CPU
uses the default address in two situations:
1. If you power up without a configuration, the default station address of 1 is used.
2. When the Port Mode parameter is set to Message Mode, and Modbus becomes the protocol
in STOP Mode, the station address defaults to 1, unless you specify a STOP Mode for the
serial port in the CPU configuration, and then change the station address to be used for STOP
Mode.
In either of these situations, if you have a slave configured with a station address of 1, confusion
may result when the PACSystems CPU responds to requests intended for that slave.
CPE400 does not support this protocol.
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