SATEC 1771DB - Manual (Page 7)

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3 GENERAL OPERATION
3.1 Interfacing to the BASIC Module
The BASIC module interfaces with the PLC processor via the PLC backplane interface. Data
is transferred through the block-transfer-read (BTR) and block-transfer-write (BTW) buffers
as 16-bit unsigned integers up to 65535.
The data to be sent to the BASIC module must be placed in the BTW buffer. The size,
configuration, and contents of the BTW buffer are discussed in Section 4.2.
The data to be sent to the processor is placed into the BTR buffer. There are two user-
selectable formats of data exchange. The structure and contents of these data sets are
provided in Appendix A and discussed in Section 4.1.
For more information concerning a programming technique when applying BTR and BTW
buffers, refer to the 1771-DB BASIC Module User's Manual.
3.2 Principles of Operation
The data transfer between the processor and the BASIC module operates differently
when you are using the driver with or without the E option.
On power-up, the driver automatically enters a polling loop and continuously polls all the
Powermeters listed in the polling list. The driver checks the response time, message integrity
and data value ranges.
If the Powermeter response is OK, then data is placed to the BTR buffer and the request to
the processor for a data transfer is issued. When the E option is used, the driver waits until a
block transfer occurs. After the processor has read data from the BTR buffer, the driver goes
to the next Powermeter. In the driver without the E option, if no data transfer occurs within
2 seconds, the driver goes to the next Powermeter.
When the driver fails to receive an acceptable response for a certain Powermeter, it retries
twice. If the driver has failed to get any response from the Powermeter, then it considers
the Powermeter "dead" and disables it. If the E option is used, then the processor is informed
of the error by a non zero error code in word 64 of the BTR buffer. For the possible error
codes, refer to Table 4.1 below. If the E option is not used, the processor is not informed of
an error, and the driver goes to the next Powermeter.
Note that disabled Powermeters are resuscitated after 3-6 minutes depending on the number
of instruments being polled. On reconnecting a Powermeter, it gets back to the polling mode.
When the reset (R) option is used, then before entering the next polling loop, the driver always
checks whether there are any writes pending in the BTW buffer. If no data transfer occurs
within 2 seconds, the driver returns to the polling loop. If there is data pending in the BTW
buffer, then the appropriate command is sent to the addressed Powermeter or Powermeters,
and driver goes back to the polling loop.