Casella CEL CEL-296 - Manual (Page 56)

Timing
Eleven bytes of data containing the currently displayed or stored
result and the status of the instruments are output every 500
milliseconds, (the same rate as the display is updated).
When this data is available for output, the "request to send" line is
set low. As the instruments use the serial data lines for other
purposes internally, the receiving hardware or software should be
enabled ONLY when the "request to send" line is low.
The instrument then interrogates the interface 64 times every
second and outputs the eleven data bytes when the "clear to send"
line is found to be low. As the interface can output 32 characters in
500 milliseconds, the "clear to send" line can also be used to hold tip
the sound level meter for short periods, either between individual
bytes or between blocks of bytes, while the receiving device
processes the data.
As the sound level meter transmits digital data regularly at 500
millisecond intervals, the timing for the receiving device can be
derived from it using 1 minute = 120 results.
Page 56 CEL-296/3 Operator Handbook CEL Instruments Ltd