Zilog Z16C30 - Manual (Page 117)

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Z16C30 USC ®
U SER ' S M ANUAL
UM97USC0100
Z ILOG
5.24 BETWEEN FRAMES, MESSAGES, OR CHARACTERS (Continued)
In sync modes, once the conditions to start sending a
message or frame (described above) are met, the Trans-
mitter may send a bit sequence called a Preamble. A
Preamble can be used to synchronize Phase Locked Loop
and decoding circuits at the remote receiver, or, for USCs
manufactured after June 1993, to guarantee a minimum
number of Flags between HDLC/SDLC frames. Whether
the Transmitter sends a Preamble is a function of the
TxMode and sometimes the TxSubMode, as follows:
TxMode Preamble sent?
Monosync If CMR13=1
Slaved Monosync Never
Bisync If CMR13=1
Transparent Bisync If CMR13=1
802.3 (Ethernet) Always
HDLC/SDLC If CMR13=1
HDLC/SDLC Loop Never
If the Transmitter sends a Preamble, the TxPreL and
TxPrePat fields of the Channel Control Register (CCR11-
10 and CCR9-8) control its length and content:
TxPreL Length of Preamble Sent
00 8 bits
01 16 bits
10 32 bits
11 64 bits
TxPrePat Preamble Pattern Sent
00 All zeroes
01 All ones, or Flags
10 101010...
11 010101...
For HDLC/SDLC mode, if TxPrePat is 01 and the
FlagPreamble bit in the Channel Control Register (CCR12,
see Figure 5-17) is 1, a USC manufactured after June 1993
sends 1, 2, 3, 4, or 8 Flags as the Preamble. Including the
opening and closing Flags, this guarantees a minimum of
3, 4, 6, or 10 Flags between frames respectively. This is
useful when sending to certain kinds of equipment that
can’t handle less Flags, or as a means of slowing the gross
frame rate slightly, perhaps as a “congestion manage-
ment” measure.
FlagPreamble should be 0 in all other modes. For 802.3
(Ethernet) mode, program TxPreL=11 and TxPrePat=10;
the Transmitter automatically modifies the last (64th) bit
from a 0 to a 1 to act as the “start bit”. For other modes,
consider the sections of Chapter 4 that deal with data
encoding and the DPLL, and whatever standards or speci-
fications apply to your application, in deciding whether to
use a preamble and if so what kind.
After sending the Preamble, or when the conditions for
starting a frame have been met if there is no Preamble,
except in 802.3 (Ethernet) mode the Transmitter sends an
opening Flag or Sync sequence. In the two Bisync modes
this may differ from the closing sequence:
TxMode Opening sequence
Monosync (TSR15-8)
Slaved Monosync (TSR15-8)
Bisync (TSR7-0)(TSR15-8)
Transparent Bisync DLE-SYN
(ASCII or EBCDIC per CMR12)
802.3 (Ethernet) None
HDLC/SDLC Flag (01111110)
HDLC/SDLC Loop Flag (01111110)
In the HDLC/SDLC and HDLC/SDLC Loop modes only, the
Transmitter will combine the closing and opening Flags
into a single instance if software has not selected sending
a Preamble (CMR13=0; this doesn’t apply in Loop mode),
and the conditions for starting a frame (described earlier in
this section) are met as the Flag is going out.
As described in the earlier section 'Status Reporting',
software can use four of the bits in the Transmit Command/
Status Register (TCSR) to track the progress of the Trans-
mitter through these inter-frame activities. They occur in
the time order CRCSent, then EOF/EOM Sent, IdleSent,
and finally PreSent. Chapter 7 describes how software can
enable any or all of these conditions to cause an interrupt.
5.24.2 Async Transmission
As described in the previous section, the TxIdle field of the
Transmit Command/Status Register (TCSR10-8) controls
what kind of idle line condition the Transmitter sends
between characters (or words) in asynchronous modes.
The bits in the Channel Command Register that define the
Preamble in sync modes (CCR11-8) can be used in Async
mode to “shave” the length of transmitted Stop bits.
UM009402-0201