ISDN TA POTS Interface

CMX625

 2001 Consumer Microcircuits Limited

11

D/625/2

Each channel consists of the following 4 bytes:

(i)

The first two bytes consist of two 64kbps data channels, labelled ‘B1’ and ‘B2’, and transfer B channel

data to and from the network.

(ii) The third byte, labelled ‘Monitor’, is used for programming and controlling devices attached to the

IOM-2 interface. The data structure within the monitor channel is not defined and will be device

specific. The CMX625 is programmed via the monitor channel (see section 1.5.15).

(iii) In digital applications (ISDN line cards) the fourth byte contains two bits for the 16kbps ‘D’ channel,

four ‘Command/Indicate’ (C/I) bits for real time status information and two handshake bits for

supporting the handling of the monitor channel, labelled ‘MR’ and ‘MX’ (monitor transmit and receive).

The handshake procedure is described in section 1.5.4. In analogue applications (analogue line

cards) there is no 'D' channel in the fourth byte so the adjacent C/I channel is increased to 6 bits. The

C/I1 channel bits are used in the same way as for Terminal Mode.

1.5.4

Monitor Channel Handshake Protocol

The Monitor channel operates on an event driven basis. While data transfers on the bus take place

synchronised to the frame sync, the flow of data is controlled by a handshake procedure using the

outgoing MX (monitor transmit) and incoming MR (monitor receive) bits. Data is placed onto the monitor

channel and the MX bit is activated. This data will be transmitted repeatedly (once per 8kHz frame) until

the transfer is acknowledged (ACK) via the MR bit. The actual data rate is not fixed but is dependent upon

the response speed of the transmitter and receiver. The protocol is applicable to both TE and non-TE

modes.

Byte 1

Byte 2

Byte 3

Byte n

ACK

ACK

ACK

ACK

125µs

EOM

MR

MX

Data

Figure 5 Monitor Handshake Timing (general case)

Figure 5 shows the general case for monitor handshake timing. 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. The next byte is

placed on the bus after the inactive-to-active transmission of MR, as early as the next frame (there is no

limit to the maximum number of frames). At the time that the second byte is transmitted, MX is returned

inactive (high) for one frame (MX inactive for more than one frame indicates an End of Message). In

response to MX going active (low), MR will be deactivated (high) for one frame (the MX inactive to MR

inactive delay can be any number of frames). This procedure is repeated for each additional byte. The

transmitter sends an End of Message (EOM), after the last byte of data has been transmitted, by not

reactivating MX after deactivating it.

The receiver can hold off the transmitter by keeping MR active until the receiver is ready for the next byte.

The transmitter will not start the next transmission cycle until MR goes inactive.

The transmitter is able to abort a transmission by holding MX inactive (high) for two or more frames, this

will generate an interrupt when the INTERRUPT MASK Register bit 3 is unmasked (logic ‘1’) and bit 3 of

the Status Register will be set to ‘1’.