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Packet type codes are used as the destination identifier

of a message, in Fermilab communications software.  

A packet type code is therefore associated with a certain
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class of messages, related to a specific function or application in
,
a system.
6
A program declares ownership of a packet type code (PTC) or several
@
PTCs, indicating that it wishes to receive all messages with that
J
PTC.  For each distinct link only one program (or sub-part of a program
T
for RT-11) may own a particular PTC at any instant.
^
Programs may send messages with any PTC.
h
Some system-wide assignments of packet type codes (PTCs) have already been made.
r
System-wide packet 
|
type code values will be used 
†
for the same function under all operating systems (RSX-11M, RT-11
�
and VMS). 
š
 Using packet type 
¤
codes in this way means that a program which sends or receives messages of
®
a particular type, 
¸
need have no knowledge of which operating system the machine
Â
at the other end of a link is running.
Ì
The messages themselves will have their own application dependent structures
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and protocols for each PTC.
à
 For example   a packet type code
ê
value of  3 is used systemwide for messages relating to 
ô
requests for  events to be sent for analysis. Any analysis program,
þ
e.g. RT-MULTI
or RSX-MULTI or any other RSX analysis program, 
sends off request messages with packet type code 3.
When the data acquisition machine is operating under RT-11, a part of the
&RT-MULTI data
0acquisition system handles these requests and 
:provides buffers of events for an analysis program
Din a linked machine.
NIt sends event buffers in reply to requests, wherever these requests
Xmay have originated.
b When the data acquisition machine is operating under RSX-11M 
lrequests for events are handled by the RSX task GTEVNT,
vwhich supplies events to analysis programs 
€either over a physical DR11-W link or pseudo-link.  The packet
Štype code used by the analysis program receiving the events may take
”any legal value.  Part of the internal format of a request message 
žwith packet type code 3 is to specify the packet type code
¨to be used for the reply.
².page 
¼.hl 1 System-wide packet type codes
Æ The following packet type code assignments have been made so far:
Ð.literal
Ú 
ä	1	Event Concatenator.
î 
ø		Receives part-events from one or more data 
		acquisition machines. Combines them together
		into a complete event, ready to be logged to tape.

 	2       Link logger.
*
4		Sends part-events on the request of an event 
>		concatenator.
H
R        3	Analysis event sender.
\
f		Receives requests for events or buffers of events 
p		and sends them to the requester.
z
„	4	RT-MULTI link analysis receiver.
Ž 
˜                Normally used as the particular packet type code for
¢		the analysis program, in the case of RT-MULTI.
¬
¶	5	Run Control
À 
Ê	6	Error Logging
Ô 
Þ	7	Remote Camac access
è 
ò	8	Remote File access
ü 
	9	Remote RSX MCR
 
.end literal
$.sk 2
..hl 1 Private packet type codes
8 Please use values between 100 and 255 for your own program's packet type codes.
BAny value may be chosen for the packet type code of messages which your
Lprogram wishes to receive, provided that it does not conflict with 
Vthose chosen by another program running at the same time on the same link.
`The values for packet type codes do not necessarily
jhave to be the same each time a program  is run.  For example, your
tprogram may make a service request to a piece of system
~software, such as requesting a block of events for analysis.
ˆThe request message must contain the packet type code you have chosen
’as the reply message packet type code.  If you are designing an
œapplication where several tasks communicate with each other, you will
¦need to use some fixed assignment of packet type codes within your
°own application.  Co-ordinated system design is needed in this case
ºto avoid conflicts in the use of packet type codes.  Both the RT-11
Äand RSX-11M drivers will ensure that only one task, or one sub-part
Îof a program may own a particular  packet type code.