Rainbow Electronics DS1957 - Manual (Page 16)

DS1957
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READ ROM [33H]
This command allows the bus master to read the DS1957’s 8-bit family code, unique 48-bit serial
number, and 8-bit CRC. This command can be used only if there is a single DS1957 on the bus. If more
than one slave is present on the bus, a data collision will occur when all slaves try to transmit at the same
time (open drain will produce a wired-AND result). The resultant family code and 48-bit serial number
will usually result in a mismatch of the CRC.
MATCH ROM [55H]
The match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a
specific DS1957 on a multidrop bus. Only the DS1957 that exactly matches the 64-bit ROM sequence
will respond to the subsequent memory function command. All slaves that do not match the 64-bit ROM
sequence will wait for a reset pulse. This command can be used with a single or multiple devices on the
bus.
SKIP ROM [CCH]
This command can save time in a single drop bus system by allowing the bus master to access the
memory functions without providing the 64-bit ROM code. If more than one slave is present on the bus
and 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).
SEARCH ROM [F0H]
When a system is initially brought up, the bus master might not know the number of devices on the
1-Wire bus or their 64-bit ROM codes. The search ROM command allows the bus master to use a
process of elimination to identify the 64-bit ROM codes of all slave devices on the bus. The ROM search
process is the repetition of a simple 3-step routine: read a bit, read the complement of the bit, then write
the desired value of that bit. The bus master performs this simple, 3-step routine on each bit of the ROM.
After one complete pass, the bus master knows the contents of the ROM in one device. The remaining
number of devices and their ROM codes may be identified by additional passes. See Chapter 5 of the
“Book of DS19xx iButton Standards” for a comprehensive discussion of a ROM search, including an
actual example.
OVERDRIVE SKIP ROM [3CH]
On a single-drop bus this command can save time by allowing the bus master to access the memory
functions without providing the 64-bit ROM code. Unlike the normal Skip ROM command the
Overdrive Skip ROM sets the DS1957 in the Overdrive Mode (OD=1). All communication following
this command has to occur at Overdrive Speed until a reset pulse of minimum 480 µs duration resets all
devices on the bus to regular speed (OD=0).
When issued on a multidrop bus this command will set all Overdrive-capable devices into Overdrive
mode. To subsequently address a specific Overdrive-capable device, a reset pulse at Overdrive speed has
to be issued followed by a Match ROM or Search ROM command sequence. This will shorten the time
for the search process. If more than one slave supporting Overdrive is present on the bus and the
Overdrive Skip ROM command is followed by a read command, data collision will occur on the bus as
multiple slaves transmit simultaneously (open drain pull-downs will produce a wire-AND result).