Special Implementation Features; Modbus/Tcp Request Queueing; Conversion Of Serial Line Parameters; Redirect Of Modbus/Tcp - VSCOM ModGate Plus User Manual

7 Special Implementation Features

7. Special Implementation Features

7.1. Modbus/TCP Request Queueing

A master on Modbus/TCP may send multiple requests at the same time, i.e. without waiting for
the response on each. This is not possible on serial lines, so the requests are placed in a queue and
served one by one. Serial ports are served from this queue independently.
As a side effect of this implementation variant multiple masters on Modbus/TCP may connect to
the same serial line. Their requests are placed into the queue in a first come - first served manner.
Multiple serial masters on Modbus/RTU may forward their requests using an extra ModGate Plus
to go to Modbus/TCP as intermediate. Then these requests use the target queue as well.

7.2. Conversion of Serial Line Parameters

On ModGate Plus models with multiple serial lines a master on a serial port can address slaves
on the other serial ports. The DirectMappingMode allows for that configuration. The involved
serial lines do not need to share the configuration parameters. The requests and their responses are
transferred from one serial line to the other by getting the logical content. This enables to change
the complete serial line configuration, including the Modbus operation mode.
So one port configured for Modbus/ASCII by 9600 bps and 7 bits on RS232 may connect to another
port operating Modbus/RTU by 115200 bps and 8 bits on RS485.
Note: the transfer from one serial port to another does not share the queue function of section 7.1.

7.3. Redirect of Modbus/TCP

Using the Direct Mapping, a ModGate Plus can receive requests via Modbus/TCP and forward
them to other TCP connections. The masters on Modbus/TCP then only need to know about the
one ModGate Plus Gateway, and not the individual target devices. However for this the masters
need to send a dedicated Modbus ID, not the default 255/0xFF.
This option is also useful, if all or many of the masters use Wireless LAN for communication, and
the target devices are on Ethernet. And of course vice versa.
June 2018 ModGate Plus User Manual 35