Rainbow Electronics DS1993 - Manual (Page 15)

DS1992/DS1993/DS1994
15 of 23 102199
HARDWARE CONFIGURATION Figure 8
TRANSACTION SEQUENCE
The protocol for accessing the DS199X via the 1-wire port is as follows:
§ Initialization
§ ROM Function Command
§ Memory Function Command
§ Transaction/Data
INITIALIZATION
All transactions on the 1-wire bus begin with an initialization sequence. The initialization sequence
consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the
slave(s). The presence pulse lets the bus master know that the DS199X is on the bus and is ready to
operate. For more details, see the “1-Wire Signaling” section.
ROM FUNCTION COMMANDS
Once the bus master has detected a presence, it can issue one of the four ROM function commands. All
ROM function commands are eight bits long. A list of these commands follows (refer to flowchart in
Figure 9):
Read ROM [33h]
This command allows the bus master to read the DS199X’s 8-bit family code, unique 48-bit serial num-
ber, and 8-bit CRC. This command can only be used if there is a single DS199X 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 DS199X on a multidrop bus. Only the DS199X 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.