DS2405
18. The master must learn the other devices’ ROM data. Therefore it starts another ROM Search sequence
by repeating steps 13 to 15.
19. At the highest bit position where the master wrote a 0 at the previous pass (step 16), it now writes a 1.
This deselects device 2, leaving device 3 active.
20. As in step 17, subsequent reads to the end of the ROM will not show bit conflicts. This completes the
fourth ROM Search pass where the master has learned another ROM’s contents.
After one complete pass, the bus master knows the contents of the 64-bit ROM in one device. Subsequent
passes will reveal the total number of devices and their individual ROM codes. In addition, after each
complete pass of the search that successfully determines the 64-bit ROM for a specific device on the
multidrop bus, that particular device can be individually accessed as if a Match ROM has been issued
since all other devices will have dropped out of the search process and are waiting for a Reset Pulse. The
DS2405 that was discovered by the search process will not toggle the state of its PIO pin at the end of the
search, but additional read time slots issued by the bus master after the search is completed will cause the
DS2405 to output the logic state of its PIO pin onto the 1-Wire bus. If the pulldown is on and the PIO pin
is a logical 0, the DS2405 will respond with read-0 time slots. If the pulldown is off and the PIO pin is a
logical 1 (external pullup is required), the DS2405 will respond with read-1 time slots. Each additional
read time slot issued by the bus master will continue to indicate the state of the PIO pin until a Reset
Pulse is received from the bus master. The combination of Match ROM and Search ROM allows the user
to change the state of the PIO pin and report the current state (Match ROM) or simply report the current
state of the PIO pin without changing it (Search ROM).
Active-Only Search ROM [ECh]
The Active-Only Search ROM command operates similarly to the Search ROM command except that
only devices with their output pulldown turned on are allowed to participate in the search. This provides
an efficient means for the bus master to determine devices on a multidrop system that are active (PIO pin
driven low). After each pass of the active-only search that successfully determines the 64-bit ROM for a
specific device on the multidrop bus with its output pulldown turned on, that particular device can be
individually accessed as if a Match ROM had been issued since all other devices will have dropped out of
the active-only search process and are waiting for a Reset Pulse. The DS2405 that was discovered by the
active-only search process will not toggle the state of its PIO pin at the end of the search, but additional
read time slots issued by the bus master after the search is completed will cause the DS2405 to output the
logic state of its internal CONTROL signal (see Figure 2) onto the 1-wire bus. Since the Active-Only
Search ROM command only operates on devices with their pulldown on, the internal CONTROL signal
for each of these parts is always a logical 1. With CONTROL=1, the selected DS2405 will respond to the
bus master with read-0 time slots after an active-only search pass is successfully completed. Each
additional read time slot issued by the bus master will continue to appear as a read-0 until a Reset Pulse is
received from the bus master. If the CONTROL signal is a logical 0 for any DS2405, that device will not
participate in the Active-Only Search. The combination of Search ROM and Active-Only Search ROM
allows the user to search in the most efficient manner depending on the requirements. If the bus master
interrogates a multidrop system comprised of DS2405s whose PIO conditions are unknown, the Active-
Only Search can quickly determine which devices are turned on. The two commands also allow the bus
master to separately determine the state of the PIO pin and the internal CONTROL signal which may be
useful in detecting certain conditions. If Search ROM returns read-0 time slots (PIO=logical 0) for a
given device, it may be due to that particular DS2405 driving its PIO pin low, or under certain conditions
the logical 0 may be caused by some other device holding PIO low. If that same device is found using an
active-only search, CONTROL must be a logical 1 and the PIO pin is being held low by the DS2405. If
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