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PEB 20321
PEF 20321
IOM
-2 Interface
Data Sheet
193
2001-02-14
8.2
The built-in IOM
-2 handler processes the C/I and MONITOR channels of the IOM
-2
protocol; i.e. 8 independent IOM
-2 channels (subframes)
.
It also supports the D-
channel access (priority control) on the S/T interface via the DRDY input line
(
Figure 72
).
IOM
-2 Handler
8.2.1
Monitor Handler
Handshake Procedure
The monitor channel is full duplex and operates on a pseudo-asynchronous basis, that
is, while data transfers on the bus take place synchronized to frame sync, the flow of data
is controlled by a handshake procedure using the MX and MR bits. For example: data is
placed onto the monitor channel and the MX bit is activated. This data will be transmitted
repeatedly (once per 8-kHz frame) until the transfer is acknowledged via the MR bit.
Thus, the data rate is not 8-kbytes/s.
Figure 74
illustrates the flow of events, and
Figure 75
and
Figure 18
show the timing.
Idle
The MX and MR pair being held inactive for two or more frames constitutes the channel
being idle in that direction. The data (logical
“
high
”
, i.e. negative logic) received in the
monitor channel is invalid and should be
“
11111111
”
.
Start of Transmission
The first byte of data is placed on the bus and MX is activated (low). MX remains active,
and the data remains valid until an inactive-to-active transition of MR is received,
indicating that the receiver has read the data off the bus.
Subsequent transmission (general case
Figure 75
).
–
In the case of the second byte
transfer the transmitter detects the MR bit transition from the inactive to the active state
before transmitting the new second byte. At the time that a new byte is transmitted, MX
is returned inactive for one frame time only (MX inactive for more than one frame time
indicates an end of message), the data is valid in the same frame. In the following frame
MX returns active again and the same data is transmitted. Data is repeated in
subsequent frame and MX remains active until acknowledgement is detected (MR
transition from inactive to active).
Only packets with a multiple of two bytes length can be transmitted via the monitor
channel (refer to registers IOMCON1 and IOMTMO).