w65c51n Western Design Center, Inc., w65c51n Datasheet - Page 25

• Manufacturer Part Number: w65c51n
• Description: Asynchronous Communications Interface Adapter Acia
• Manufacturer: Western Design Center, Inc.

DIAGNOSTIC LOOP-BACK OPERATING MODES
A simplified block diagram for a system incorporating an ACIA is shown in Figure 18.
It may be desirable to include in the system a facility for “loop-back” testing, of which there are two
kinds.
1.
2.
The ACIA does not contain automatic loop-back operating modes, but they may be implemented with
the addition of a small amount of external circuitry. Figure 19 indicates the necessary logic to be used
with the ACIA.
Essentially, LLB = high does the following:
1. Disables outputs TxD, DTRB and RTSB (to Modem)
2. Disables inputs RxD, DCDB, CTSB, DSRB (from Modem)
3. Connects transmitter outputs to respective receiver inputs (i,e., TxD to RxD, DTRB to DCDB, RTSB
to CTSB).
LLB may be tied to a peripheral control pin (from a W65C21 or W65C22S, for example) to provide
processor control of local loop-back operation. In this way, the processor can easily perform local
loop-back diagnostic testing.
Remote loop-back does not require this circuitry, so LLB must be set low. However, the processor
must select the following:
1. Control Register bit 4 must be 1, so that the transmitter clock equals the receiver clock.
2. Command Register bit 4 must be 1 to select Echo Mode.
3. Command Register bits 3 and 2 must be 1 and 0, respectively to disable IRQB interrupt to
transmitter.
4. Command Register bit 1 must be 0 to disable IRQB interrupt for receiver.
In this way, the system re-transmits received data without any effect on the local system.
Loop-back from the point of view of the processor. In this case, the Modem and Data Link must
be effectively disconnected and the ACIA transmitter connected back to its own receiver, so that
the processor can perform diagnostic checks on the system, excluding the actual data channel.
Remote Loop-back
Loop-back from the point of view of the Data Link and Modem. In this case, the processor, itself,
is disconnected and all received data is immediately retransmitted, so the system on the other
end of the Data Link may operate independent of the local system.
Local Loop-Back
The LBB line is the positive-true signal to enable local loop-back operation.
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