Datasheet

SCBA015A
7–18
Fast GTLP Backplanes With the GTLPH1655
BTL Bus
The specification of the BTL bus was conceived especially for large backplane systems. The
basic circuit layout of a BTL bus is shown in Figure 9.
Driver
Receiver
Z
O
R
TT
= Z
O
BTL Bus
V
REF
= 1.55 V
+2.1 V
Figure 9. Circuit Concept of BTL Bus
The outputs of a BTL driver are provided with open-collector pins. The maximum capacitance of
an I/O pin was fixed at 5 pF. To attain this goal, a diode is connected in series with the output
transistor. The series connection of the capacitance of the transistor with the capacitance of the
diode results in a reduction of the total capacitance. This circuit results in a low level of 1 V.
To allow switching of the bus lines with the incident wave, the specification for the drive
capability was fixed at I
OL
= 100 mA.
The high level is generated using a terminating resistor connected to 2.1 V at the end of the line.
With bidirectional lines, a termination resistor must be provided at both ends of the line. As a
result of the low signal amplitude of 1.1 V, the bus line can be terminated correctly with BTL
systems. The maximum output current (I
OL
= 100 mA) allows driving a terminating resistor of as
low as 11 Ω (1.1 V/100 mA). If the BTL driver is in the middle of the bus line, the lower limit for
the impedance is 22 Ω. This is sufficient for all bus systems that are used, particularly when the
impedance and the signal speed are kept high, as a result of the limitation in the I/O
capacitance.
By definition, the threshold voltage is 1.55 V, exactly midway between the low and high levels.
In large systems, it is essential that it be possible to remove and reinsert boards during system
operation (live insertion). To meet this requirement, the precharge function has been
implemented in BTL circuits. By means of this function, the capacitance of the pin, the stub line,
and the I/O pin on the insertable board can be charged to the threshold voltage (1.55 V) before
this pin comes in contact with the signal line on the backplane. Thus, it is possible to prevent
signals on the backplane wiring from being so seriously interfered with that the data is corrupted.
The most serious disadvantage of the BTL bus is its high power consumption. If the transistor of
a driver stage is operated to the limits of its specification, at the low level, a current of 100 mA
can flow, with a voltage drop of 1 V. This results in the output stage dissipating 100 mW. If a
16-bit bus driver is used, in the worst case, 1.6 W may be consumed in the output transistors
alone. With small surface-mounted components, this power consumption makes it necessary to
use packages with a special heat sink.