Texas Instruments 990 User Manual page 246

Communications Structures
D.4.2 Packet with No User Data
When a control packet is transmitted over the data link the
format appears on the line as follows:
2280627
where:
Flag
Addr
I FIELD ,
FLAG
ADDR.
I
I
I
CONTROL PACKET
I
CRC
FLAG
is the start-of-frame and end-of-frame flag , >7E.
is the switch address of the secondary station.
I field includes only the control packet, that is, a receive
ready, a reset request, a reset confirmation, or a
receive not-ready packet.
CRC is the frame check character.
D.5 TASK LEVEL IDENTIFIERS
The three levels of X.2S are the physical link, the data link
procedures and the packet level procedures. To provide for task
to task, or program to program, communications a supplimentary
method 6f addressing is available. This method provides for
addressing uniquely .:.dentified programs or devices in the
distributed environment. The addresses are refered to as
Destination IDs and Source IDs. There is a unique ID assigned to
TM990 and PM550 secondary stations. In data packets arriving at
the secondary stations the ID is refered to as the destination
ID. In data packets sent from the secondary station the same ID
is refered to as the source ID. The IDs must be included in
every data packet entering and leaving the devices. To send a
data packet to the primary station, TM990 and PM550 do the
following after receiving the first input data packet:
1. Include the session control word in the output data
packet.
2. Take the destination and source IDs received in the
input data packet and reverse their relative positions
for the output data packet. In other words, make the
source ID in the received data packet the destination
ID in the send data packet, and vice-versa.
D-12 2270526-9701