ICS ELECTRONICS 4809A, Instruction Manual

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Communication with the Modbus device is serial and requires that the user set
the 4899A and the Modbus device to the same serial settings. Each Modbus
device has its own address so that it can respond to serial packets sent to its
address. The Modbus device address is different and independent from
the 4899A's GPIB bus address setting. Although the typical Temperature
Chamber has only one Modbus Controller, the 4899A can drive multiple
Modbus devices when using a RS-485 network. The 'C' command is used
to set the Modbus device address in the 4899A. The 4899A remembers the
Modbus device address until changed by a subsequent 'C' command or the
4899A is powered off or reset.
Modbus devices are register based devices and they are controlled by writing
values to registers that control different functions i.e. temperature setpoint,
alarm settings etc. Data is taken from Modbus devices by reading registers
associated with those parameters i.e. temperature, humidity, etc. ICS has
created a set of simple Modbus commands for reading, writing and com-
municating with Modbus devices. When these Modbus commands are sent
to the 4899A over the GPIB bus, the 4899A communicates with the selected
Modbus device. Modbus commands should not be mixed or concatenated
with IEEE-488.2 or SCPI commands.
If the 4899A's message packet is successfully received by the Modbus
device, the Modbus device will generate a response packet that either
confirms receipt of the message or that contains the requested data. The
4899A receives the response packet and validates the packet. If the response
packet is a valid response to a read command, the returned data is held in
the GPIB transmit buffer and will be output on the GPIB bus the next time
the 4899A is addressed to talk. If the message is an acknowledgment mes-
sage, there is no further action.
The 4899A expects to receive a response from the Modbus device within
a preset time period or it declares a timeout error. The timeout period is
programmable and is factory set to 100 milliseconds. It is better to set the
timeout period to a larger than needed value to avoid unnecessary timeout
errors.
If the message was not a valid message, or was an exception message, or
was missing, then the 4899A sets the appropriate bit(s) in the Questionable
Condition Register and puts a decimal value in the Modbus Error register.
Both registers are part of the 4899A’s Status Reporting Structure. If the