Rockwell Automation T6200 Compressor Anti-Surge and Capacity Controller - Manual (Page 193)

Modbus Interface – RS-232/RS-485
B-3
T6200 MODBUS Configuration
The T6200 supports communications with foreign devices using the MODBUS communications
protocol. The user interface is through global tags which reside in the T6200 (configured as
remote tags). The T6200 communications port may be configured for MODBUS as a Master or a
Slave device using the PORT command.
The data points configured with tag names and optional scaling for REAL data points. The
following table describes the format of the PORT command for MODBUS communications
configuration:
PORT …..
{1}
{2400, 4800, 9600, 19200, 38400}
{N,O,E}
{7,8}
{1,2}
{RS232,
RS-485}
MODBUS [RTU, ASCII]
{MASTER, SLAVE} …”;”
The PORT directive introduces the MODBUS configuration. As a MODBUS master, the T6200
will poll the port for current data. As a MODBUS slave, it will respond with the same data on the
port.
The logical link definition specifies the baud rate, parity (None, Odd, Even), number of
data bits (must be 8 for RTU), number of stop bits, and electrical line type.
The protocol, , is MODBUS RTU or ASCII Hex. RTU is a binary protocol which uses
a 16-bit Cyclic Redundancy Check (CRC) algorithm to verify data integrity. In this mode, parity
is usually not required. ASCII Hex protocol uses an LRC check algorithm. The characters are
ASCII and may be easily displayed on a terminal.
The MODBUS role, , is either MASTER or SLAVE. In the master role, the T6200
polls the foreign unit(s). In the slave role, the T6200 is polled by the foreign master; it responds as
one or more MODBUS units. In the master role, polling parameters, , may be
specified here and on the unit statement.
“@”{0.1 .. 60}
EXCEPTION
GAP
MAX
OFFSET {0,1}
PRIORITY {0,..2}
RETRY {0..99} [“/” {0..60}
The polling parameters, , determine the polling characteristics of a MODBUS master
as well as the possible offset between point addresses and on-the-wire addresses (OFFSET). Only
OFFSET and EXCEPTION apply to MODBUS slaves.
“@” Polling interval in seconds
Exception handling control flags.
2 Exception on bad poll address
4 Limit response quantity to poll quantities
8 No response on bad poll address
Number of unused coils or inputs which may be
polled to poll succeeding coils or inputs.
Number of unused holding or input registers which
may be polled to poll succeeding values.