SLUS441A – MARCH 1999 – REVISED AUGUST 2000
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
DIFFB: DIFFSENS input pin. Connect through a 20-k
resistor to DIFSENS and through a 0.1-
to ground. Input to comparators that detect what types of drivers are connected to the SCSI bus.
DIFSENS: SCSI bus DIFSENS line driver.
DISCNCT: Disconnect pin. Shuts down the terminator (switches terminating resistors off the bus) when tied to
GND. The disconnect pin high (or OPEN) enables the terminator.
GND: Power supply return.
LINEn-: Line termination pins. Negative line in differential pair.
LINEn+: Line termination pins. Positive line in differential pair.
LVD: (28-pin package only) Indicates that the bus is in LVD mode.
REG: Regulator bypass pin. Bypass near the terminator with a 4.7-
F and a high-frequency, low-ESR 0.01-
capacitor to ground.
2.75 V to 5.25 V supply. Bypass near the terminator with a 4.7-
F and a high-frequency, low-ESR
F capacitor to ground.
All SCSI buses require a termination network at each end to function properly. Specific termination requirements
differ, depending on which types of SCSI driver devices are present on the bus. The UCC5681 is a low-voltage
differential (LVD)-only device. It senses which types of drivers are present on the bus. If it detects the presence
of a single-ended (SE) or high-voltage differential (HVD) driver, the UCC5681 will place itself in a
high-impedance input state, effectively disconnecting the chip from the bus.
The UCC5681 senses what kinds of drivers are present on the bus by the voltage on SCSI bus control line
DIFFSENS, which is monitored by the DIFFB input pin. The DIFSENS output pin on the UCC5681 attempts to
drive a DIFFSENS control line to 1.3 V. If only LVD devices are present, the DIFFSENS line will be successfully
driven to that voltage. If HVD drivers are present, they will pull the DIFFSENS line high. If any single-ended
drivers are present, they pull the DIFSENS line to ground (even if HVD drivers are also present on the bus). If
the voltage on the DIFFB is below 0.5 V or above 2.4 V, the UCC5681 enters the high-impedance SE/HVD state.
If it is between 0.7 V and 1.9 V, the UCC5681 enters the LVD mode. These thresholds accommodate differences
in ground potential that can occur between the ends of long bus lines.
Three UCC5681 ICs are required at each end of the SCSI bus to terminate 27 lines (18 data, 9 control). Every
UCC5681 contains a DIFSENS driver, but only one should be used to drive the line at each end. The DIFSENS
pin on the other devices should be left unconnected.
On power up (the voltage on the TRMPWR pin rising above 2.7 V), the UCC5681 assumes the SE/HVD mode.
If the voltage on the DIFFB input indicates LVD mode, the chip waits 100 ms to 300 ms before changing the
mode of the bus. If the voltage at the DIFFB input later crosses one of the thresholds, the UCC5681 again waits
100 ms to 300 ms before changing the mode of the bus. The magnitude of the delay is the same when changing
in or out of either bus mode. A new mode change can start anytime after a previous mode change has been
The DIFFB inputs on all three chips at each end of the bus should be connected together. Properly filtered, noise
on DIFFB will not cause a false mode change. There should be a shared 50-Hz noise filter implemented on
DIFFB at each end of the bus as close as possible to the DIFFB pins. This is implemented with a 20-k
between the DIFFB and DIFSENS pins, and a 0.1-
F capacitor from DIFFB to ground. See the Typical
Application diagram at the end of this datasheet.