ICS ELECTRONICS 9099, Инструкция по эксплуатации

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3-2
3
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. The 9099 can
handle integer and floating point values. ICS has a set of Modbus commands
for writing data to and reading data from Modbus devices which are listed in
Table 3-5. When one of these Modbus commands is sent to the 9099, the
9099 sends the appropriate Modbus RTU packet to the selected Modbus device.
Modbus commands should not be mixed or concatenated with IEEE-488.2 or
SCPI commands.
If the 9099'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 9099 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 and will be transmitted to the cli-
ent the next time the 9099 is sent a device_read request If the message is an
acknowledgment message, there is no further action.
The 9099 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 300 milliseconds.
If the message was not a valid message, or was an exception message, or was
missing, then the 9099 sets the appropriate bit(s) in the Questionable Condi-
tion Register, puts a decimal value in the Modbus Error register and sets bit
6 in the ESR Register. Both registers are part of the 9099’s Status Reporting
Structure shown in Figure 1. If the appropriate register enable bits are set
true, then the Service Request bit will be set and generate a device_intr_srq
message (SRQs). If a Reverse Interrupt Channel exists, the device_intr_srq
messages are sent to the Application over the Reverse Interrupt channel. The
user can then query the Status Byte to determine the cause of the Service Re-
quest. Refer to Application Bulletin AB80-4 for information on setting up a
VXI-11 Reverse Channel.
3.2.3 Operation with the VXI-11 Protocol
The VXI-11 protocol is a higher level Ethernet protocol that provides a pseudo
GPIB interface. In addition it offers certain capabilities that allow simultaneous
access by multiple clients at the same time without the clients interfering with
each other (via LOCK and UNLOCK). Also note that the VXI-11 protocol
being an RPC service means it supplies high level error handling and error