SP505AN Datasheet, PDF (4/30 Pages) Sipex Corporation – Multi-Protocol Serial Transceivers

DCE Configuration to a DB-25 Serial Port
The SP505, SP506 and SP507 can also be easily
configured as a DCE in all serial communication
applications. Figure 1 summarizes the usual signals
used in synchronous serial communications.
However when sourcing the signal by the DCE, the
transceiver must be configured as a driver. The basic
configuration shown in Figure 3 illustrates the
connection to a DB-25 D-sub connector.
Programmable DTE/DCE Configuration
to a DB-25 Serial Port
The SP505, SP506 and SP507 can also be conveniently
configured so that the interface is programmable for
either DTE or DCE. Extra attention must be paid
to the direction of the signals since there may be
bidirectional signals present. Figure 4 and 5 illustrate
a connection to a DB-25 D-sub connector using the
SP506 and SP507, respectively.
When bidirectional signals are needed, this usually
means a driver and receiver are half-duplexed together.
In other words, the driver outputs are connected to
the receiver inputs. This requires the driver outputs
to be disabled and at a high impedance state.
The receiver does not require a disable function as
long as the inputs are high enough impedance
so that the driver signals are not attenuated.
A half-duplexed receiver without a disable function
will still produce a signal at its output when the
driver is active and communicating with the receiver
at the other end of the cable. This signal can be ignored
unless the receiver output is tied to the driver input.
If this is the case, then the receiver output should a
buffered with a latch or 2:1 mux in order to direct the
driver input or receiver output into the HDLC device.
The SP507 has additional receivers with enable lines
for easier DTE/DCE implementation.
The SP505, SP506 and SP507 can be configured on
the equipment as either DTE or DCE to the DB-25
connector. For the illustration on Figure 4, DTE is
used with the SP506. Since only a DB-25 connector
is used as the equipment's serial port, daughter
cables are still needed for the other connector types.
In addition, to support DCE on this serial port,
crossover cables are used. Thus, the equipment will
need to provide a DTE V.35 cable and a DCE V.35
cable, for example.
Crossover cables merely reroute the signals to the
appropriate connector pin assignment. For DTE in
V.35 mode, pins P and S are used for Transmit Data
(ITU#103), and pins R and T are used for Receive
Data (ITU#104). Pins P and S are connected to the
driver outputs since they are sourced from the DTE.
Pins R and T are connected to the receiver inputs
since they are sourced for the DCE. To convert the
serial port to a DCE configuration, the crossover
cable swaps the signals to those pins. Specifically,
the DB-25 will have pins 2 and 14 connected to the
driver and pins 3 and 16 connected to the receiver.
This is a normal DTE allocation. However, by the
time these signals reach the other end of the cable to
the ISO2593 V.35 connector, the pins 2 and 14 now
go to R and T, respectively. Pins 3 and 16 on the
DB-25 side now go to pins P and S, respectively.
Therefore, pins R and T are now generating the data
and thus, connected to the driver output. Similarly for
pins P and S, now connected to the receiver inputs.
The configuration on Figure 5 uses the SP507 in a
popular DTE/DCE configuration. The TxC signal is
half-duplex and bidirectional. The DCE_ST driver is
active during DCE mode while the DTE_ST receiver
is active during DTE mode. The STEN and SCTEN
enable lines are connected together for common
DCE/DTE control. Similarly with the RL/DCD pair
and the LL/TM pair. The DCD signal is used for this
driver labelled RL in this case. The Remote Loopback
function is not available in this configuration.
The same goes for the Test Mode function where the
TM receiver is used for Local Loopback when in
DCE mode.
On-Board Programmable
DTE/DCE Configuration
(Without Crossover Cables)
DTE/DCE programmability can also be achieved
without using crossover cables. Instead, the selection
can be designed in the circuitry. This requires a
bidirectional serial port for all signals, not just TxC
and DCD. An "on-board" solution would need to
have circuitry allocated for DTE and circuitry
allocated for DCE. The transceiver portion would
need to address disable functions, low leakage
currents, and specific timing issues when joined
together in a half-duplex configuration.
SP505/6/7APN/03
SP505, SP506, SP507 Application Note
4
© Copyright 2000 Sipex Corporation