CD1865 Datasheet, PDF (81/150 Pages) Intel Corporation – Intelligent Eight-Channel Communications Controller

Intelligent Eight-Channel Communications Controller — CD1865
are Local and Remote Loopback. Used together with diagnostic firmware in the host system, the
Loopback Modes provide very thorough test coverage of all CD1865 functional blocks: the
CD1865 processor, ROM, RAM, bus interface, transmitters/receivers, and random logic.
Local Loopback Mode
Local Loopback mode is a ‘silent’ loopback, for example, data being sent by the transmitter is
internally connected to the receiver without reaching the external TxD pin. Generally, this is
advantageous because it allows diagnostic software to operate without causing unwanted effects on
any remote device that may be connected to the serial line. During local loopback, the TxD pin is in
the ‘mark’ (a logic ‘1’) state. If non-silent loopback is also needed, it can be easily implemented
externally with an AND gate or a jumper plug on the serial connector.
Local Loopback mode is invoked by setting the LLM bit in the Channel Option register 2 (COR2)
and then issuing a channel command to tell the CD1865 that COR2 has changed. When in this
mode, the channels TxD Output is internally looped back to the channel’s RxD Input. However, all
other channel parameters including modem pins continue to work independently and normally.
Receive special character recognition, overflow handling, and other options may be tested by using
the Local Loopback mode and transmitting the appropriate character sequences. As shown in
Figure 27 on page 82, the loopback connection is directly from the TxD signal to the RxD signal,
for example, all transmit and receive logic is tested except the actual I/O buffers.
Remote Loopback Mode
Remote Loopback mode is provided to support testing of devices connected to the serial lines.
Remote Loopback is invoked by setting the RLM bit in the Channel Option register 2 (COR2).
When in this mode, the CD1865 echoes the received data to the transmitter for transmission back to
the sender. The received data is not passed on to the host.
When in Remote Loopback mode, the transmitter continues to run as defined by its own Baud Rate
registers, not the values being used by the receiver. The CD1865 receives a complete character,
strips off Start, Stop, and Parity bits, and then re-transmits it with Parity, Length, and Stop bit
Output options as defined in COR1. Thus, it is possible to change baud rate. However, this can
result in receiver overflow. In general, when programming for Remote Loopback Operation, the
Transmit bit rate should be as fast or slightly faster than the expected receive rate to avoid possible
overrun and loss of data. The number of Stop bits should be set to a one, rather than one-and-a-half
or two, if the application permits it. This ensures that the effective transmit rate is faster than the
receive rate.
As shown in Figure 27, Remote Loopback is done at the character level and not the bit level. The
Receive and Transmit FIFOs are not used in Remote Loopback. Characters are transferred directly
from the Receive Holding register to the Transmit Holding register. For a diagnostic mode that
tests the FIFOs, other logic is needed to be implemented by programming the host system to
transfer received characters from the Receive FIFO to the Transmit FIFO. This permits full testing
of FIFO thresholds, service request logic, special character operation, and so on.
Datasheet
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