Datasheet

SN65HVD252
SN65HVD253
www.ti.com SLLSE37 –JUNE 2010
APPLICATION INFORMATION
USING THE SILENT MODE
The silent mode is selected by setting a logic high on pin 8 (S). In silent mode, the driver function of the
transceiver is disabled, whereas the receiver function remains active. This silent mode may be used to
implement babbling idiot protection, to ensure that the driver does not disrupt the entire network in case of a local
fault. The silent mode may also be used in redundant systems to select or de-select the redundant transceiver
until needed.
USING THE AUTOBAUD FEATURE OF THE SN65HVD253
The autobaud feature of the HVD253 is selected by placing a logic high on pin 5 (AB). In autobaud mode, the
normal bus-transmit function of the transceiver is disabled, whereas the bus-receive function and all of the other
normal operating functions of the device remain active. An internal loopback emulates the connection between
the driver outputs and the receiver inputs, allowing the receiver to respond to locally-generated dominant bits as
well as dominant bits from other nodes.
With the autobaud function engaged, normal bus activity, including activity from the local controller, can be
monitored by the local node as received data. However, if an error frame is generated by the local CAN
controller, it is not transmitted to the bus. Only the local microprocessor can detect the error frame.
Autobaud detection is well suited to applications that have a known selection of baud rates. For example,
DeviceNet (a common industrial protocol) has optional signaling rates of 125 kbps, 250 kbps, or 500 kbps. Once
a logic high has been applied to pin 5 (AB) of the HVD253, the local controller may assume a baud rate such as
125 kbps and then wait for a message to be transmitted by another node on the bus. If the wrong local signaling
rate has been selected, an error message is generated by the local CAN controller. However, because the
bus-transmit function of the transceiver has been disabled, no other nodes receive the error message from the
local controller.
This procedure makes use of the CAN controller status-register indications of message received and error
warning status to signal if the current signaling rate is correct or not. The warning status indicates that the CAN
controller error counters have been incremented. A message-received status indicates that a good message has
been received.
If an error is generated, the local CAN controller may assume another signaling rate and wait to receive another
message. When an error-free message has been received, the correct baud rate has been selected. A logic low
may now be applied to pin 5 (AB) of the HVD253, returning the bus-transmit normal operating function to the
transceiver.
USING THE V
REF
OUTPUT
The V
REF
output provides a stable voltage of half the power supply voltage. This can be used in split-termination
schemes to improve electromagnetic compatibility (EMC) of the system. It can also be used as a reference with
which to compare the single-ended inputs for degraded operation in the event of a wire-break fault on either the
CANH or CANL bus lines.
DeviceNet REQUIREMENTS
DeviceNet requires additional performance beyond the requirements of the ISO 11898-2 CAN standard. These
additional specifications address the conditions found in rugged industrial applications. The DeviceNet
specifications are maintained by ODVA. (www.odva.org) The HVD252 and HVD253 fully meet these
requirements under all recommended operating conditions.
PARAMETER DeviceNet SPECIFICATION HVD252, HVD253
Number of nodes 64 Yes
Minimum differential input resistance 20 kΩ Yes
Minimum differential input capacitance 24 pF Yes
Bus pin voltage range (survivable) –25 V to 18 V Yes
Bus pin voltage range (operation) –5 V to 10 V Yes
Differential output voltage 1.5 V with 50-Ω load Yes
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