Tektronix 2221 - Page 90

u
Options and Accessories - 2221 Operators
Table 7-13
Parity
Selectiona
When CR ( normal mode ) is selected as the terminator ,
the instrument will :
Switch
Position
Accept only
CR as
the line
terminator
. Parity
Type
Comment
6
7
•
Send
CR
as
the
last
byte of a
message
.
оо
ODD
The parity bit of each byte is set
or cleared as needed to make the
number of
logical ones per
word
byte
odd .
When CR - LF is selected as the terminator , the instru
ment will
:
1
EVEN
Accept either CR - LF or LF only as the line terminator .
The parity
bit of each
byte is set
or cleared as needed to make the
number
of
logical ones per
word
byte even .
• Send CR - LF ( carriage return followed by line feed ) at
the
end
of
every
message .
MARK
The parity
bit is
always set to a
logical one .
1 1 SPACE
STOP Bits
The parity bit is always cleared to
a logical zero .
aCharacters are
always
accepted if
possible
.
If parity is
enabled and AQS is on , a status byte is sent if the received
parity doesn't match the parity selected . Parity must be dis
abled (
PARAMETERS switch
position 5 set to
o before
power
on ) for binary data
transfers .
Once communication is established between the
controller
and
the
oscilloscope ,
commands may be
sent to
the oscilloscope . When dealing with the transfer of data
via the RS -
232 - C
interface , the bits
used to make
up a
character
consist of a start
bit ,
seven or eight
data bits ,
and , finally , one or two stop bits . Start and stop bits
separate the data bytes and are called framing pulses . The
start bit is always set to a “ mark , " and the one or two
stop bits are set to a " space . " One stop bit is used in
most applications . Two stop bits may be needed for some
printers at some baud rates . The command STOP 1 or
STOP 2 sets the number of stop bits in the char
acter
frame
.
UU
Message Line Terminator . PARAMETERS Switch
section 8 selects the line terminator . The line terminator is
either
CR
(
carriage
return
) ,
with
switch
section
8
open , or
CR - LF ( carriage return and line feed ) , with switch section 8
closed (
see
Table
7-11
) .
NOTE
NOTE
For the 2220 and 2221 instruments , selection of the
stop bits is not possible from the front - panel con
trols .
When
connecting to a
printer
or
plotter
with a
choice
of
stop bits
for
different baud - rate
settings
,
select
a
baud
rate
that
requires only
one stop bit .
U
Commands to the
oscilloscope are
interpreted and
carried out
as soon as
they are
recognized as such ;
the oscilloscope does not wait for a CR or CR - LF to
end the command string . If a command needs to be
correctly done before the next command is sent , the
controller
must wait for the
correct
return
. If an
error
occurs ( due
to
command syntax or
incompatible
instrument settings ) , the error status will be
immediately reported . The controller can detect the
error , query the event code , and take corrective
action before going on with another command that
may not be
handled
properly
. This is
especially
true
if the
previous command puts the
oscilloscope in a
state that prevents it from responding . For this
reason , the recommended practice is to send only
one
command
in
each
message
line
to
the
oscilloscope
.
The
transition from
one
character's stop bit ( s
) to the
next
character's start
bit is used
to
synchronize the
receiver to the transmitter . This causes the coded data
bits for each character to be read at the best time relative
to the start of
the
character's start .
Errors that
occur due to
mismatched baud rates ,
data
bits , or stop bits show up as " framing errors . " The start - bit
and stop - bit frame surrounding the character bits have the
wrong timing relationship with respect to each other . Since
they are not recognized properly , the data stream cannot
be
interpreted by the
receiving
device .
7-14