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PK-232 TECHNICAL MANUAL
4.7.4.2
Persistence: CTL = $02
PERSISTENCE (P) and SLOTTIME work together to establish the time at which the PK-
232 will transmit.
OERSICTENCE is best dexcribed by quoting Phil Karn, KA9Q, directly from his "Raw TNC
Functional Spec".
"Whenever the host has queued data for transmission, the TNC begins monitoring the
carrier detect signal from the modem. It waits indefinitely for this signal to go inactive.
Once the channel is clear, the TNC generates a random number between 0 and 255. If
this number is less than or equal to P, the TNC asserts the transmitter PTT line, waits
.01 # TXDELAY seconds, and transmits all frames in its queue. The TNC then releases
PTT and goes back to the idle state. If the random number is greater than P, the TNC
delays .01 # SLOTTIME seconds and repeats the procedure. If the carrier detect signal
has gone active in the meantime, the TNC again waits for it to clear before continuing.
Note that P=255 means always transmit as soon as possible, regardless of the random
number.
The result is that the TNC waits for an exponentially-distributed random interval afr
sensing that the channel has gone clear before attempting to transmit. The idea here is
that with proper tuning of the parameters P and SLOTTIME, several stations with traffic
to send are much less likely to collide with each other when they simultaneously see
the channel go clear."
The form of the command is:
$C0
FEND CTL
n
where
can be 0-255; the persistence parameter p is the fraction: (n + 1)/256
n
If
= 0,
n
If
= 255,
4.7.4.3
SLOTTIME: CTL = $03
The SLOTTIME command controls the slot interval as described above in the PERSIS-
TENCE command.
The form of the command is:
$C0
FEND CTL
n
where
is the slot interval in units of 10 milliseconds.
4.7.4.4
TXTAIL: CTL = $04
In both the PK-232's human or verbose mode and the original Host mode, the length of
time between the end of data and the release of the PTT line is determined automati-
cally by the baud rate (HBAUD).
In the KISS protocol, the computer controls this interval.
PK232TM Rev. A 5/87
n
$02
$C0
p
FEND
p = 1/256
p = (155 + 1)/256 = 1.0
n
$03
$C0
SLO
FEND
CHAPTER 4 – HOST MODE AND SPECIAL APPLICATIONS
(lowest value)
(highest value)
4-12
Page 29
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