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response. The typical action for the Model 8099 is to transfer data to or from
the Serial Interface.
The 8099’s Serial Interface has two sets of drivers and receivers. One set pro-
duces or receives the RS-232 single ended, bipolar signals. Typical output
levels are ± 8 Vdc. Minimum input levels are ±5 Vdc. The second set gener-
ates and receives differential RS-485 signals. The RS-485 driver has more
drive capability than the older RS-422 drivers but it makes the same signal
levels. The two pairs of signals are connected together externally by the user
to form the common RS-485 two-wire network. The Tx/Rx signal pair can
be jumpered to an available termination network on the serial connector. The
8099’s RS485 parameter must be set on so the 8099 will not drive the RS-485
network when it is not transmitting a message.
Incoming serial data from the Modbus slave device is received, converted into
TTL levels by the desired receiver chip and applied to the UART in the ARM
processor. Each received character is temporarily stored in the serial received
data buffer. The characters in the received message are counted and verified
against the expected response character count. The message is then check
summed. If the received message is a valid response, any data is converted in
to the correct format and placed in the output buffer where they can be trans-
ferred out when the client requests them (Analogous when a GPIB device is
addressed to talk) Messages that contain errors or Exception messages cause
the 8099 to set bits in the Questionable Register and to place an error value in
the Modbus Error Register. The 8099 contains a multilevel Status Byte Register
and Event Register structure enables the 8099 to generate a Service Request
when errors are detected.
Flash Memory contains all of the 8099's program instructions, command tables,
and power turn-on/self test routines. At power turn-on, the 8099 performs a
self test on each functional block to determine whether there is a gross system
failure. Any self test error is displayed as a pattern of blinking LEDs on the
front panel. The error pattern is repeated until the unit is turned off. The RDY
LED comes on to indicate a successful completion of the self test routine.
The 8099's configuration settings, serial number and other parameters that are
subject to change are saved in a nonvolatile Flash sector. At power on time,
the microprocessor copies the saved configuration to RAM where it is used to
operate the unit. Any changes made to the settings during run time are not stored
in the Flash sector until the user sends the 8099 the *SAV 0
command.