DS1481 Datasheet, PDF (3/10 Pages) Dallas Semiconductor – 1.Wire Bus Master with Overdrive

DS1481
When OD = 1, communication with the 1–wire device is
exactly as described in the operation section above.
The actual 1–wire timing for both modes of operation is
described in Figures 4, 5 and 6.
Note that when toggling the OD bit there is no change on
the I/O line.
PRINTER COEXISTENCE
In order to coexist with parallel port printers, the DS1481
utilizes two input pins (I1 and I2) and two output pins
(O1/BSY1 and O2/BSY2). When ENI is low these pins
are used for transmitting data received on the I/O pin or
for issuing an unmistakable busy signal. When ENI is
inactive (high) O1/BSY1 and O2/BSY2 propagate the
states of I1 and I2.
If a printer is attached to a DS1481, I1 is connected to
the printers BUSY signal (low only if printer is on line and
busy), and I2 is connected to SELECT OUT (driven low
if printer is off line), see Figure 2.
If the attached printer is “powered up” and on line, the
DS1481 uses SELECT OUT for communication regard-
less of the state of the printers BUSY signal. If the
printer is off line its BUSY signal is inactive (high) and
this line is used by the DS1481 for host communication.
If the attached printer is powered off, both SELECT OUT
and BUSY will be low. This prevents meaningful com-
munication with the DS1481 because it is unable to de–
assert its busy signal (O1/BSY1 and O2/BSY2 low) or
return a high sample of the I/O pin.
To solve this problem, the DS1481 uses the busy signal
issued during a reset to detect the presence of another
DS1481 based device attached behind it on the parallel
port. If this busy signal is not detected by the DS1481, it
assumes that it is the last DS1481 based device on the
port.
If the DS1481 determines that it is the last device on the
port it ignores the states of its I1 and I2 pins while ENI is
low. It also leaves the ENO pin high to prevent sending
line feed signals to the printer. This gives the last device
the ability to control O1/BSY1 and O2/BSY2 without
affecting stackability.
EPP/ECP TRANSPARENT MODE
When the DS1481 first powers up it is in a transparent
mode in which the three signal lines (auto line feed, busy
and select) that pass through the part are directly con-
nected by transmission gates. This allows bi–direc-
tional printers (or other parallel port peripherals) to com-
municate in either the EPP or ECP mode of the PC
parallel port. The DS1481 pin sets connected are as fol-
lows:
ENI
O1/BSY1
O2/BSY2
ENO
I1
I2
1–wire communication using the DS1481 is impossible
in transparent mode. To toggle to normal mode four
consecutive overdrive toggle commands must be
issued. If this sequence has been issued and the ENI
pin remains high for at least 10 ms the part will enter its
normal mode of operation. Note that any other 1–wire
time slot command issued during the sequence resets
the sequence. The steps needed to return to transpar-
ent mode are as described above with the exception
that no additional wait is required at the end of the four
overdrive toggles to enter transparent mode.
While in transparent mode if the DS1481 detects that
the ENO pin has been held low for more than 10 ms it
turns off the transmission gate connecting ENI and
ENO. This guarantees the host will have the ability to
take the DS1481 out of transparent mode and perform
1–wire I/O operations.
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