If more than one slave is present on the bus and, for example, a read command is issued following the Skip ROM command, data collision will occur on the bus as multiple slaves transmit simultaneously (open drain pull-downs will produce a wired-AND result).
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DS2406
ROM FUNCTIONS FLOW CHART Figure 13
Bus Master TX
Reset Pulse
DS2406 TX
Presence Pulse
R
Bus Master TX ROM
Function Command
S
33h
55h
F0h
ECh
CCh
N
N
N
N
N
Read ROM
Match ROM
Search ROM
Conditional
Skip ROM
Command
Command
Command
Search
Command
?
?
?
?
?
Y
Y
Y
Y
Y
Condition
Fulfilled ?
DS2406 TX Bit 0
DS2406 TX Bit 0
DS2406 TX
Master TX Bit 0
DS2406 TX Bit 0
DS2406 TX Bit 0
Family Code
(1 Byte)
Master TX Bit 0
Master TX Bit 0
Bit 0
Bit 0
Bit 0
Match ?
Match ?
Match ?
DS2406 TX Bit 1
DS2406 TX Bit 1
DS2406 TX
Master TX Bit 1
DS2406 TX Bit 1
DS2406 TX Bit 1
Serial Number
(6 Byte)
Master TX Bit 1
Master TX Bit 1
Bit 1
Bit 1
Bit 1
Match ?
Match ?
Match ?
DS2406 TX Bit 63
DS2406 TX Bit 63
DS2406 TX
Master TX Bit 63
DS2406 TX Bit 63
DS2406 TX Bit 63
CRC Byte
Master TX Bit 63
Master TX Bit 63
Bit 63
Bit 63
Bit 63
Match ?
Match ?
Match ?
R
Bus Master TX Memory
(See Figure 7)
Function Command
Vertical
Spare
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DS2406
Conditional Search ROM [ECh]
The Conditional Search ROM command operates similarly to the Search ROM command except that only devices fulfilling the specified condition will participate in the search. This command provides an efficient means for the bus master to identify devices in a multidrop system that have to signal a status change, e.g. the opening of a window in a building control application.
The condition is specified by the bit functions CSS0 to CSS4 in Status Memory location 7. At power-on all these bits are 1s. They can be changed by means of the Write Status command. As long as the device remains powered up, the modified search conditions are available for use at any time. For the conditional search, one can specify the polarity (HIGH or LOW; CSS0), the source (PIO-pin, channel flip flop or activity latch; CSS1, CSS2) and the channel of interest (A, B or the logical OR of A, B; CSS3, CSS4).
Table 2 shows all qualifying conditions and the required settings for CSS0 to CSS4.
QUALIFYING CONDITIONS FOR CONDITIONAL SEARCH Table 2
DESCRIPTION
CONDITIONAL SEARCH SELECT CODE
CHANNEL SELECT
SOURCE SELECT
POLARITY
CONDITION
CHANNEL
CSS4
CSS3
CSS2
CSS1
CSS0
RESERVED
Don’t care
0
0
0/1
Unconditional
neither one
0
0
At least one of these
0
bits needs to be 1
Activity Latch = 0
A
0
1
0
1
0
Activity Latch = 1
A
0
1
0
1
1
Channel FF = 0
A
0
1
1
0
0
(transistor on)
Channel FF = 1
A
0
1
1
0
1
(transistor off)
PIO Low
A
0
1
1
1
0
PIO High
A
0
1
1
1
1
Activity Latch = 0
B
1
0
0
1
0
Activity Latch = 1
B
1
0
0
1
1
Channel FF = 0
B
1
0
1
0
0
(transistor on)
Channel FF = 1
B
1
0
1
0
1
(transistor off)
PIO Low
B
1
0
1
1
0
PIO High
B
1
0
1
1
1
Activity Latch = 0
A or B
1
1
0
1
0
Activity Latch = 1
A or B
1
1
0
1
1
Channel FF = 0
A or B
1
1
1
0
0
(transistor on)
Channel FF = 1
A or B
1
1
1
0
1
(transistor off)
PIO Low
A or B
1
1
1
1
0
PIO High
A or B
1
1
1
1
1
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DS2406
The activity latch (Figure 11) captures an event for interrogation by the bus master at a later time. This way, the bus master needs not interrogate devices continuously. The activity latch is set to 1 with the first negative or positive edge detected on the associated PIO channel. It can be cleared with the Channel Access command if the ALR bit of the Channel Control Byte 1 is set. The activity latch is automatically cleared when the DS2406 powers up. In order to use the activity latch the output transistor of the selected channel should be non-conducting. Otherwise signals applied to the PIO pin will be shorted to ground by the low impedance of the output transistor.
The Channel Select bits CSS3 and CSS4 specify the channel of interest. The sampling of the source within the selected channel will take place on completion of the Conditional Search command byte. The Channel selection codes are as follows:
CSS4
CSS3
Channel Selection
0
0
neither channel selected
0
1
channel A only
1
0
channel B only
1
1
channel A OR channel B