• Serial buffer: The serial port buffer, which is declared in SerialOpen(), must
be large enough to hold all data a device will send. The buffer holds the data
for subsequent transfer to variables. Allocate extra memory to the buffer
when needed, but recognize that memory added to the buffer reduces final-
data memory
(p. 515).
Note Concerning SerialInRecord() running in pipeline mode with NBytes
(number of bytes) parameter = 0:
For the digital measurement sequence to know how much room to allocate in
Scan() buffers (default of 3), SerialInRecord() allocates the buffer size specified
by SerialOpen() (default 10,000, an overkill), or default 3 • 10,000 = 30 kB of
buffer space. So, while making sure enough bytes are allocated in SerialOpen()
(the number of bytes per record • ((records/Scan)+1) + at least one extra byte),
there is reason not to make the buffer size too large. (Note that if the
NumberOfBytes parameter is non-zero, then SerialInRecord() allocates only this
many bytes instead of the number of bytes specified by SerialOpen()).
• Variable Declarations — Variables used to receive data from the serial
buffer can be declared as Public or Dim. Declaring variables as Dim has the
effect of consuming less telecommunication bandwidth. When public
variables are viewed in software, the entire Public table is transferred at the
update interval. If the Public table is large, telecommunication bandwidth
can be taxed such that other data tables are not collected.
• String Declarations — String variables are memory intensive. Determine
how large strings are and declare variables just large enough to hold the
string. If the sensor sends multiple strings at once, consider declaring a single
string variable and read incoming strings one at a time.
The CR1000 adjusts upward the declared size of strings. One byte is always
added to the declared length, which is then increased by up to another three
bytes to make the length divisible by four.
Declared string length, not number of characters, determines the memory
consumed when strings are written to memory. Consequently, large strings
not filled with characters waste significant memory.
7.9.17.5.6 Demonstration Program
CRBasic example Receiving an RS-232 String (p. 254) is provided as an exercise in
serial input / output programming. The example only requires the CR1000 and a
single-wire jumper between COM1 Tx and COM2 Rx. The program simulates a
temperature and relative humidity sensor transmitting RS-232 (simulated data
comes out of COM1 as an alpha-numeric string).
254
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