Casella CEL CEL-414 - Manual (Page 41)

On the CEL-493/3, bytes 3 to 5 are also
used as output flags to mark the start
and finish of an SEL event. “SE” is out-
put at the start of an event, while “FE” is
output at the finish. These are coded in
standard ASCII as follows.
BIT 7 6 5 4 3 2 1 Ø
Ø Ø X X X X X X
Where Byte 3 = S (=Ø1Ø1ØØ11),
Or = F (=Ø1ØØØ11Ø),
Byte 4 = E (=Ø1ØØØ1Ø1),
Byte 5 = Space.
The following half-second outputs for
bytes 3 to 5 occur in SEL mode.
Where SE = Start of event,
XXX = Changing SEL value
displayed on LCD
during event,
FE = End of event,
YYY = SEL value of whole
event (will not change
until a new event
starts or SEL is reset),
SE = Start of next event
(will be a minimum
of one sec after the
end of the previous
event, so that each
SEL value is output
at least once).
In addition to the serial SEL output, there
is a 5 V CMOS digital output on line 14 of
the auxiliary socket to control tape
recorders , etc. This line is normally 0 V,
but during events, it is pulled up to 5 V.
Interface Connection
Only the first code byte has the MSB set,
so the receiver can always synchronise
with the five bytes by checking for this bit.
The serial bit stream is transmitted with
the least significant bit (i.e. bit Ø) first,
then bit 1 etc.
The interface uses four pins on the auxil-
iary socket in the base of the instrument
(or of any CEL filter set coupled to it). The
interface should not be connected di-
rectly to RS 232 level devices as all four
lines are at CMOS levels (0 & 5 V).
Two of the lines are used for serial data
transmission, while the other two are for
handshaking (see Table 2).
Voltage
levels:
0 & 5 V nominal (logic 0
& 1 respectively),
Baud rate: 4800 baud,
Start bits: 1,
Data bits: 8,
Stop bits: 2,
Parity: None.
Table 2 Connector Pins Used in Interface
Pin Direction Function Description
2 — Serial ground 0 V
10 Output Serial data Serial data output from sound level meter.
9 Output Request to send This line is set low by the sound level meter
when data is available for output. Data is
valid ONLY while this line is low.
12 Input Clear to send This line must be set low by the receiving
device to enable output from the sound level
meter.
General CEL Sound Level Meters Handbook - Page 37