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the pipeline. Responses to later, or other, requests may become out of sync and the
master may match the wrong response with a request. The resulting symptoms could
range from appearing to receive wrong data from a slave, to slaves going off-line, to
a gradually slowing down of response time.
Another aspect of Modbus/TCP that must be considered relates to the Unit ID field.
Modbus/RTU incorporates the slave address field but Modbus/TCP replaces the slave
address field with a Unit ID field. TCP/IP uses IP addresses to find specific nodes on
the network. By using the IP address to specify the node, and the Unit ID to specify a
Modbus/RTU device attached to that node, multiple slaves can be located at a single
IP address. Unfortunately, some manufacturers of early Modbus/TCP systems
assumed only one slave per IP address and always used a Unit ID of 00. If this early
equipment is used, only one Modbus/RTU slave can be connected to an MES1A/B.
(The MES1B is capable of RS-485 multidrop, allowing multiple slaves.)
HINTS AND TIPS
A few simple suggestions that may assist you if your system is experiencing problems
include:
Slow down the polling rate.
Try the
Disable Pipeline
option in the MES1A/B configuration menu.
Make sure your Modbus/TCP equipment uses sequence and Unit ID
numbers.
