Tektronix 77807-1, User Manual

Page: 71 / 132

Appendix B: Remote Control
B–8
DS1000 Series User Manual
If the byte 10 hex is part of the data, this byte is sent twice so that it is
not confused with DLE ETX (10 hex), which ends the data phase.
Polling for Status Messages
If a television demodulator has a message, it does not send it to the
PC, because more than one television demodulator can be connected
to the same remote interface bus. To avoid data contention, the PC
uses polling to check for messages.
Polling is done with the PATH? command. PATH? returns an empty
string if the television demodulator has no message, and returns the
path if a message is available. The path contains the addresses Ad and
Ars (see Addresses on page B–3).
The following example is one continuous polling communication
between the PC and the television demodulator. The values for Ad,
Ars, and Arr can be calculated according to instructions on page B–3.
The PC addresses a television demodulator that is busy, repeats the
addressing phase until the television demodulator returns a ready
phase, then the PC sends the “PATH?” command.
PC > [DLE] [ENQ] [Ad] [Ars]
Send Addressing phase
DS1000 >[DLE] [WACK]
Not Ready response
PC > [DLE] [ENQ] [Ad] [Ars]
Send Addressing phase
DS1000 > [DLE] [ACKO] [Ad] [Ars]
Ready response
PC > [DLE] [STX] ['PATH?'] [DLE] [ETX]
Data phase
After receiving the PATH? command, the television demodulator
places its answer in its transmit buffer. The PC tries to retrieve the
message with a receive cycle, but the television demodulator has not
processed the answer yet.
PC > [DLE] [ENQ] [Ad] [Arr]
Receive Addressing phase
DS1000 > [DLE] [WACK]
Not Ready phase
The PC repeats the addressing phase until the television demodulator
is ready and transmits its data phase. There is no data included,
because the television demodulator has no message.
PC > [DLE] [ENQ] [Ad] [Arr]
Receive Addressing phase
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