4
4-1
4
Theory of Operation
4.1
INTRODUCTION
This section describes the theory of operation of the 8099 Parallel Interface
cards.
4.2
FUNCTIONAL DESCRIPTION
The 8099 is a microprocessor based device that performs the VXI-11 service
functions to control its Serial Interface from an IEEE-802.11 network. The
8099 is made up of seven major elements, most of which are interconnected to
the microprocessor by a common data, address and control signal bus.
Incoming Ethernet messages are received by the LAN Interface chip. Each
received message passes through a TCP Stack before being processed. If the mes-
sage is a Modbus command then it is converted into a series of binary characters
to make up the Modbus RTU message packet. The Modbus message packet,
shown in Figure 4-1, includes the slave device address, the command number,
the registers and data (if any) that is being sent to the registers. A checksum
is added to make up the complete Modbus RTU packet. The Modbus packet
is then placed in the serial transmit buffer. From the serial transmit buffer, the
data characters are sequentially placed in the microprocessor's UART where
they are serialized, passed through the selected serial driver and outputted at
the serial interface.
Packet Format: Addr Cmd Registers.... .data CRC
Figure 4-1 Modbus RTU Packet
All Modbus packets are responded to by the Modbus device. The response
packet is either an acknowledgement, an error message or response data.