ProSoft Technology MVI69-GSC - Manual (Page 79)

MVI69-GSC ♦ CompactLogix or MicroLogix Platform Reference
Generic ASCII Serial Communication Module User Manual
ProSoft Technology, Inc. Page 79 of 102
September 24, 2010
The receive buffer in the module can hold up to 4096 characters. This large size
permits the buffering of a large amount of data before a transfer of the data to the
controller is required. The module buffers incoming ASCII characters in its
receive buffer until one of the user-specified termination conditions is recognized.
The module will then transfer the received terminated string to the controller.
The ladder logic required to properly handle transfer of terminated strings longer
than 50 characters per port is more complex than the simpler version of ladder
logic discussed above. If the terminated string is larger than 50 characters,
multiple blocks will be used to transfer the data to the controller. The first block
will contain a value of -1 in the "Number of Characters Received" data field. This
indicates that there will be more blocks to follow and that the current block
contains 50 ASCII character codes. As long as more than 50 characters remain
in the buffer waiting to be sent to the ladder logic, successive Read Blocks will
continue to show the "Number of Characters Received" as -1. When 50 or fewer
ASCII characters remain in the buffer, the module will send the last block with a
positive number in the length field. The value passed represents the number of
characters present in the data area, which is the last characters of the complete,
terminated string. The ladder logic must recognize the presence of one or more
successive blocks with -1 lengths and then the positive number of the last block
as indication that a single, complete, long string has been completely transferred.
The two byte values at bytes 1 (port 1 transmit count) and 29 (port 2 transmit
count) inform the processor of the number of ASCII characters transferred in the
last write block to the respective port transmit buffers. If a value of zero is
returned in one of these words and data was sent in the last write block, the
ladder logic must re-send the data in the next write block because the port was in
a busy state and could not transmit the last data to be written at the time the
Write Block was receive by the module from the ladder logic. If a non-zero value
is returned in one of these bytes, the value represents the number of ASCII
characters from the last write block that were successfully moved into the port’s
transmit buffer.
The status information transferred in the read block can be used by the
processor to determine the state and "health" of the module and the device(s)
attached to each application port. An important member of the value in the status
object is error word for each port. This value contains the configuration error flags
for each port and the receive buffer overflow error flag.