RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
5-6
1070
5.2.4 Flag Transmission and Reception
The modem transmitter sends at least one flag as the opening flag of the first frame. As long as the user does not load the
8-bit transmit data register, DBUFF (register 10h), with data, the modem sends continuous flags with no zero-sharing (i.e.,
0111111001111...). This facilitates transmission of the preamble as specified in T.30. Thus, the transmitter defaults to
transmitting time-fill and, therefore, keeps the receiving link station active.
To assist the user in transmitting more than one flag between frames or at the end of the final frame, a counter can be
accessed through modem diagnostics. This counter is decremented directly in the signal processor's RAM. This means that
the number written will only last for one group of flags. For example, FSK should have at least two beginning flags for the
first frame and at least two ending flags for the final frame. However, frames between these two require only one flag. This is
why the counter is decremented directly and one flag is transmitted as a default. Diagnostics should be setup as shown
below:
ADD1 =
85h
AREX1 =
0
BR1 =
0
CR1 =
1
WRT1 =
1
The value to write into YDAM1 and YDAL1 should be 1 less than the number of flags desired. This value can be written
anytime after the RX bit returns to zero and before FLAG is set by the modem.
Using the FSK example above, assume three flags are to be transmitted at the beginning of the first frame and at the end of
the final frame.
1. Turn on RTS.
2. Wait until the RX bit is reset by the modem.
3. Disable diagnostics 1 (reset ACC1).
4. Setup diagnostics 1 as above, write 00h into YDAM1, and write 02h into YDAL1.
5. Enable diagnostics 1 (set ACC1).
6. Wait until B1A is set before resetting WRT1.
7. For the ending flag of the final frame, immediately after loading in the final byte of data or after setting EOF, again setup
diagnostics 1 as above, write 00h into YDAM1, and write 02h into YDAL1.
Another method exists for sending extra flags. The host must simply do nothing since flags are transmitted as the default
condition. In other words, after the final zero in a flag is transmitted, the modem looks to see if the host has loaded new data
into DBUFF (B2A is reset). If no new data is loaded before this time, another flag is sent. Therefore, if more than one flag is
desired, the host must wait N-1 multiples of eight bit times after FLAG is set by the modem to load new data into DBUFF,
where N is the number of flags. The host then has seven bit times in which to load new data and thus prevent another flag
from being sent. For example, if three flags are desired between frames, the host must wait at least 16 bit times and not
more than 23 bit times after FLAG is set by the modem.
As the default condition, the modem receiver continually searches for the flag data pattern. When one or more flags are
detected, the interface memory status bit FLAG (09:0) is set. The flags themselves are not presented to the host through the
DBUFF register. Therefore, as soon as a flag is observed, the modem examines the next byte of received data. If it is a flag,
an abort/idle sequence, or a FCS, it is not given to the user. Instead, the appropriate status bits are set or reset.
The modem also has the capability to detect consecutive flags with zero-sharing.
5.2.5 Information Field Transmission and Reception
For information field transmission, the host should wait for CTSP (0F:1) to transition high. The host must then load the data
into DBUFF and then wait for the data available bit B2A (1E:3) to be set by the modem before loading in the next byte of
data. If AEOF is 0 and the next byte is not loaded into DBUFF within the next eight bit times, the modem will set OVRUN
(09:7), indicating an underrun condition has occurred. To tell the modem that the host wants to end the frame, the host must
set EOF as soon as the modem has taken the last byte of the frame (B2A sets). When the modem recognizes EOF being
high, the modem will reset EOF and will transmit the FCS and closing flag. Once the host sets EOF, the host may load in the
Summary of Contents for RFX144V24-S23
Page 197: ......
