UM-0085-B09
DT80 Range User Manual
Page 359
RG
Reading Fixed Width ASCII
In this case a simple serial sensor continuously transmits a stream of records which consist of an
A
character followed by
four 4-digit fixed point (2 decimal place) temperature values (
2209
represents 22.09, for example).
This job samples the stream every 30 seconds and logs the values it reads.
BEGIN"SPOT"
PS=RS232,1200,7,E,1
RA30S
1SERIAL("\eA%4d[1CV]%4d[2CV]%4d[3CV]%4d[4CV]",W)
1..4CV(.01,"~degC",FF2)
LOGON
END
Notice:
the receive buffer is cleared at the start of the control string. The
1SERIAL
channel will therefore wait until the next update
from the sensor. An alternative strategy would be clear the buffer at the end, in which case the
1SERIAL
channel would immediately
get what it needs from the buffer. However the data it reads will be the first record in the buffer and would therefore be up to 30s old.
Reading Binary Data
In this example an even more simple sensor outputs 6 bytes of data in response to a digital signal going low. These are
to be interpreted as two binary values. The first is a 16-bit integer sequence number, in big endian format (most
significant byte first). The second value is a 32-bit voltage measurement, scaled such that 0x00000000 represents -
17.0V and 0xFFFFFFFF repr17.0V. For historical reasons, this value happens to be returned in little-endian
(least significant byte first) format.
This job triggers a reading (by pulsing the
1D
output low) every 5 seconds and reads and logs the received values.
BEGIN"RAMBUTAN"
PS=RS232,115200
S1=-17,17,0,4294967296"V"
RA5S
1DSO(100,R)=0
1CV(W)=-1
1SERIAL("%2b[1CV]%b[5CV]%b[6CV]%b[7CV]%b[8CV]",W)
1CV("Seq")
2CV(S1)=8CV*17CV*65536+6CV*256+5CV
LOGON
END
Note the following points about this job:
•
In this case the sequence number can be read as a single binary number, using
%2b
, but the measured value
must be read byte by byte and reassembled into a single value.
•
A span (
S1
) is used to scale the reading into the correct range.
•
1CV is set to an error value (-1) before each attempt to read the serial channel. If the attempt fails (e.g. no data
is forthcoming from the device) then 1CV will be unchanged, so the value -1 will be logged for the sequence
number. This makes it easy to identify the reading as invalid.
Note:
CVs can only precisely store integers with absolute value less than 16,777,216 (24 bits) – above that they will be rounded. In the
above example this is not a problem because the value is scaled and rounded anyway.
If, however, you need to recover all 32 bits exactly (for example if they represented 32 separate logic states) then you
should read them using two 16 bit conversions and log each half separately, e.g.:
BEGIN"WHISTLE"
RA1+E
1..2CV(W)=-1
1SERIAL("%2b[1CV]%2b[2CV]",W)
1CV("MS 16 bits") 2CV("LS 16 bits")
LOGON
END
(This example assumes the data word is in big endian format).