
MC68302 Applications
D-56
MC68302 USER’S MANUAL
MOTOROLA
Figure D-27. Using CD (Sync) in the NMSI Receive Case
Figure D-28 shows an external loopback example which illustrates how the receive and
transmit cases function together. RTS is externally connected to CD (sync), and CTS is a
don't-care since software operation (DIAG1 = 1 and DIAG0 = 1) is chosen.
If you combine the 6.5 cycle TCLK delay on transmission with the CD (sync) signal being
provided on the second bit of the frame (–1.5 clock delay (see Figure D-27), then the first
byte stored in the receive buffer is an $FF (i.e., 6.5 — (–1.5) = 8 clock delay).
Figure D-28. External Loopback with RTS Connected to CD
D.8.6 Other NMSI Modes
Two transparent mode variations are available for use with an NMSI: BISYNC mode and
TRANSYNC mode. These modes are described in detail in the following paragraphs.
D.8.6.1 BISYNC MODE.
You can solve the problem of demarcating frames if you can af-
ford to exclude two special characters from the data. This, of course, is only possible when
you can define the protocol at both ends of the communication link. Normal BISYNC mode
can be used to do this. Normal BISYNC mode has the advantage of being able to store an
odd number of bytes in the receive buffer, rather than words. A disadvantage, however, is
lower serial performance.
RXD
NEW
FRAME
(INPUT)
RCLK
(I/O)
CD (SYNC)
(INPUT)
CD SHOULD BE LATCHED HERE ON
THE RISING RCLK OF THE SECOND
BIT OF THE FRAME.
NO EFFECT; ONLY
THE FALLING EDGE
IS DETECTED.
EXSYN = 1
NTSYN = 1
DIAG1-DIAG0 BITS = 00
DON'T
CARE
DON'T
CARE
FRAME ENDS BECAUSE ENTER
HUNT MODE COMMAND WAS
GIVEN HERE.
FIRST BIT OF DATA RECEIVED INTO BUFFER
RXD
NEW
FRAME
(INPUT)
RCLK
(I/O)
(SYNC)
(INPUT)
CD SHOULD BE LATCHED HERE ON
THE RISING RCLK OF THE SECOND
BIT OF THE FRAME.
NO EFFECT; ONLY
THE FALLING EDGE
IS DETECTED.
EXSYN = 1
NTSYN = 1
DIAG1-DIAG0 BITS = 00
DON'T
CARE
DON'T
CARE
FRAME ENDS BECAUSE ENT
HUNT MODE COMMAND WAS
GIVEN HERE.
FIRST BIT OF DATA RECEIVED INTO BUFFER