XRT4000 Datasheet, PDF (24/46 Pages) Exar Corporation – Universal Multiprotocol Serial Interface

XRT4000
APPLICATIONS INFORMATION
Traditional interfaces either require different
transmitters and receivers for each electrical
standard, or use complicated termination
switching methods to change modes of
operation. Mechanical switching schemes,
which are expensive and inconvenient, include
relays, and custom cables with the terminations
located in the connectors. Electrical switching
circuits using FETs are difficult to implement
because the FET must remain off when the
signal voltage exceeds the supply voltage and
when the interface power is off.
The XRT4000 uses innovative, patented circuit
design techniques to solve the termination
switching problem. This device includes internal
circuitry that may be controlled by software to
provide the correct terminations for V.10
(RS423), V.11 (RS422), V.28 (RS232), and V.35
electrical interfaces. The schematic diagrams
given in Figures 17 and 18 conceptually show
the switching options for the high-speed receiver
input and transmitter output terminations
respectively. Additionally, Tables 4 and 5
provide a summary of receiver and transmitter
specifications respectively for the different
electrical modes of operation.
V.10 (RS423) Interface
Figure 19 shows a typical V.10 (RS423)
interface. This configuration uses an
unbalanced cable to connect the transmitter
TXA output to the receiver RXA input. The “B”
outputs and inputs that are present on the
differential transmitters and receivers contained
in the XRT4000 are not used. The system
ground provides the signal return path. The
receiver input resistance is 10 kΩ nominal and
no other cable termination is normally used for
the V.10 mode.
V.11 (RS422) Interface
Figure 11 shows a typical V.11 (RS422)
interface. This configuration uses a balanced
cable to connect the transmitter TXA and TXB
outputs to the receiver RXA and RXB inputs
respectively. The XRT4000 includes provisions
for adding a 125 Ω terminating resistor for the
V.11 mode. Although this resistor is optional in
the V.11 specification, it is necessary to prevent
reflections that would corrupt signals on high-
speed clock and data lines. The differential
receiver input resistance without the optional
termination is 20 kΩ nominal.
V.28 (RS232) Interface
Figure 19 shows a typical V.28 (RS232)
interface. This configuration uses an
unbalanced cable to connect the transmitter
TXA output to the receiver RXA input. The “B”
outputs and inputs that are present on the
differential transmitters and receivers contained
in the XRT4000 are not used. The system
ground provides the signal return path. The
receiver “B” input is internally connected to a 1.4
V reference source to provide a 1.4 V threshold.
The receiver input resistance is 5 kΩ nominal
and no other cable termination is normally used
for the V.28 mode.
V.35 Interface
Figure 21 shows a typical V.35 interface. This
configuration uses a balanced cable to connect
the transmitter TXA and TXB outputs to the
receiver RXA and RXB inputs respectively. The
XRT4000 internal terminations meets the
following V.35 requirements. The receiver
differential input resistance is 100 Ω ± 10 Ω and
the shorted-terminal resistance (RXA and RXB
connected together) to ground is 150 Ω ± 15 Ω.
The transmitter differential output resistance is
100 Ω ± 10 Ω and the shorted-terminal
resistance (TXA and TXB connected together)
to ground is 150 Ω ± 15.
The junction of the 3 resistors (CMTX) on the
transmit termination is brought out to pins 76
and 81 for TX1 and TX2 respectively. Figure 21
shows how capacitor C having a value of 100 to
1000 pF bypasses this point to ground to reduce
common mode noise. This capacitor shorts
current caused by differential driver rise and fall
time or propagation delay miss-match directly to
ground. If it was not present, the flow of this
current through the 125 Ω resistor to ground
would cause common mode voltage spikes at
the TXA and TXB outputs.
Rev. 1.00
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