The server connections setting on this page does not apply in this mode - the
connection count is established automatically by the TCP Device Map.
IMPORTANT: Set timeout to something long enough for the device. If too short, the
gateway will not wait long enough for a response from the Modbus server, and the
result will be a lot of "no response" errors from the device even though the device is
perfectly functional.
IMPORTANT: The timeout you set in your Modbus RTU master must be longer than the
timeout set on this page. If the TCP server queried times out, it will be tabulated as a
no-response error here in the gateway, but the gateway will return an exception 11 to
the RTU master telling it that the TCP server timed out. If your RTU master timeout is
shorter than the timeout set above, then your master will not receive that exception
report. Yet, that exception report will still be sent upon timeout, and may then result
in a collision on the network if the RTU master has started sending the next request
before waiting to receive the exception reply to the previous request. This known
collision will take on the appearance of further communication problems including
further no-response errors and possibly CRC errors. These will be avoided with timeout
settings that do not conflict.
5.2 Create TCP Device Map
The gateway can present one or more TCP servers as one or more RTU slaves. There
can be a one to one mapping, or a many to one mapping. If the TCP device is itself
another gateway, then it may contain multiple slaves within its one TCP network
address. The most common application will be a one to one mapping of RTU address
to TCP address.
The TCP Device Map is where you establish your association between RTU address and
TCP device IP address. When operating in this mode, if an RTU address is not found in
this table, then the gateway will simply not respond. The expected behavior for RTU
slave devices is that they remain silent if the request does not contain their address
(aka slave ID or unit number). This gateway may be acting as multiple RTU slaves, but
it will remain silent if the RTU address is not found in this table.
A typical one to one mapping of single RTU address to TCP address is illustrated below.
5. Accessing TCP Devices from RTU
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