to 31 Modbus RTU/ASCII slave devices without relay; this mode is mainly used for the
communication between the Modbus TCP master station and the Modbus RTU/ASCII slave
station.
When the serial port works in slave mode, the serial port can be connected to a Modbus
RTU/ASCII master device; this mode can be used for the following applications:
(1) Realize data communication between Modbus TCP client and Modbus RTU/ASCII
master station;
(2) Realize data communication between Modbus RTU/ASCII master stations;
(3) Realize Modbus TCP client and a Modbus RTU/ASCII master to communicate
with a Modbus RTU/ASCII slave at the same time.
3.1.2 Gateway working mode
The gateway has two working modes, "transparent" and "mapping", and it is in
"transparent" mode under factory settings. There is no data buffer in the "transparent
transmission" mode, and there is no need to edit the slave address mapping table. After
receiving the command from the Modbus TCP client, the gateway directly sends the
command to the Modbus RTU/ASCII slave device and waits for the slave device to respond
After the slave device responds, it directly returns the data to the TCP client. The "mapping"
mode adopts the data buffer mode, and the slave station address mapping table needs to be
edited. After the gateway is powered on, the slave stations are polled and the data is stored
in the data buffer area. After the gateway receives the command from the Modbus TCP
client, it reads the data from the data buffer area and returns the data to the TCP client. This
method can greatly reduce the waiting time when the client accesses the slave station and
increase the refresh rate.
3.1.3 Gateway data storage area
The data storage is divided into five parts, the first part is the "coil" (DO) storage area
with a total of 8192 points. The second part is the "discrete input" (DI) storage area with
8192 points in total. The third part is the "input register" (AI) storage area, with a total of
2048 words. The fourth part is the "holding register" (AO) storage area with a total of 2048
words, and the fifth part is the "system diagnosis" storage area, which stores the working
status of the slave device, with a total of 263 words. Visit the "system diagnostic area" to
obtain slave station information, which can be used to set slave station disconnection alarms