MTC-20172-PC AMI Semiconductor, Inc., MTC-20172-PC Datasheet - Page 73

• Manufacturer Part Number: MTC-20172-PC
• Description: S Interface Circuit for ISDN (SIC)
• Manufacturer: AMI Semiconductor, Inc.

6.6 M-Channel + MX +
The M-channel is a byte oriented
channel, where each byte is presented
by the transmitter and acknowledged by
the receiver. The bytes are organized in
messages.
Use and Compatibility
The M-channel should not be used in the
basic mode. The M-channel is an
additional feature.
For compatibility, a special BUSY bit
is present in the M-channel, see 7.4.
Content of the M-Channel
The content is explained in chapter 7.
The length of all MTC-20172 SIC
messages is 2 bytes.
Half-Duplex Operation
The GCI standard allows the two M-
channels to work only half-duplex. This
half duplex operation is implemented
to reduce the internal buffer capacity
of the M-transceiver.
Thus, the MTC-20172 SIC can NOT
send and receive the M-channel
simultaneously. After each received
message (of exactly 2 bytes) the M-
transceiver changes direction, if a
message must be sent in the opposite
way.
Bidirectional Operation:
Message priority
The M-transceiver gives priority to
outgoing messages, to avoid internal
buffer overflow.
The controller on the M-channel must
acknowledge the messages leaving the
MTC-20172 SIC before the MTC-
20172 SIC is able to receive any new
message.
Failing to acknowledge the message
from the MTC-20172 SIC results in a
DEADLOCK: the MTC-20172 SIC waits
for the acknowledge of the outgoing
message, and in the meantime refuses
to acknowledge the next incoming
MR : Physical Layer
message.
Operation Speed
The operation speed of the M-channel is
variable: the devices can delay the
transfer by retarding the acknowledge
or by delaying the delivery of
subsequent bytes.
The MTC-20172 SIC will adjusts to any
lower speed on the channel, but it can
transfer one M-channel byte in 250 µs
(both RX and TX). However, this speed
is limited by the half duplex operation,
and by the idle which must follow each
double byte message.
Fig. 6.11 : SDL Diagram of the M-Channel Receiver
63
MTC-20172
Data Sheet & Reference Manual
Rev. 1.1 February 1997
6.6.1 M-Channel Activity
The M-channel can only work when the
GCI interface has its clock and frame
running.
6.6.2 M-Channel Format, Bit and
Byte Numbering Convention
The M-channel is a byte oriented
channel. Bytes (also called octets) are
transmitted in ascending numerical
order. Within a byte the most significant
bit is transmitted first.