Communication among RCX computers
Four RCX Code programs are running on four independent RCX
computers. By means of the IR communication the four RCX computers
play tones in turn. This is achieved by means of a protocol on top of
the IR communication primitives in RCX Code:
Each RCX computer has a
unique identity, id = 1,2,3 and 4. The RCX computer with id = 1 starts
by sending a message with the value 2. This is interpreted by the RCX
computer with id = 2 as a message telling it to play. The computer
with id = 2 then sends a message with the value 3 telling the computer
with id = 3 to play and so on until the message with the value 1 is
sent out by the RCX computer with id = 4. This completes one round.
The programs for the RCX with id = 1 looks as follows:
<![CDATA[
Private Sub ID1_Click()
'VAR(1) = RCX ID
'VAR(2) = Message Received
'VAR(3) = Next ID
PB.BeginOfSub 1 ' SendMessage ( Next ID )
PB.SendPBMessage 0, 3
PB.EndOfSub
PB.BeginOfSub 2 ' WaitMessage
PB.SetVar 2, 15, 0 ' VAR(2) = PBMessage
PB.While 0, 2, 2, 2, 0 ' Wait for PBMessage
PB.SetVar 2, 15, 0
PB.EndWhile
PB.ClearPBMessage
PB.EndOfSub
PB.BeginOfTask 0
PB.ClearPBMessage
PB.SetVar 1, 2, 1 ' RCX ID = 1
PB.SetVar 3, 2, 2 ' Next ID = 2
PB.PlaySystemSound 2
PB.GoSub 1 ' SendMessage (Next ID)
PB.Loop 2, 9 ' Loop 9 times
PB.GoSub 2 ' WaitMessage
PB.While 0, 2, 3, 0, 1 ' While Message not my RCX ID
PB.GoSub 2 ' WaitMessage
PB.EndWhile
PB.Wait 4, 100
PB.PlayTone 440, 30
PB.GoSub 1 ' SendMessage (Next ID)
PB.EndLoop ' EndLoop
PB.GoSub 2 ' WaitMessage
PB.While 0, 2, 3, 0, 1 ' While Message not my RCX ID
PB.GoSub 2 ' WaitMessage
PB.EndWhile
PB.PlaySystemSound 5
PB.EndOfTask
End Sub
]]>
The programs for the RCX with id=2, id=3 and id=4 are similar except
for the id. The id=2 program looks as follows:
<![CDATA[
Private Sub ID2_Click()
'VAR(1) = RCX ID
'VAR(2) = Message Received
'VAR(3) = Next ID
PB.BeginOfSub 1 ' SendMessage ( Next ID )
PB.SendPBMessage 0, 3
PB.EndOfSub
PB.BeginOfSub 2 ' WaitMessage
PB.SetVar 2, 15, 0 ' VAR(2) = PBMessage
PB.While 0, 2, 2, 2, 0 ' Wait for PBMessage
PB.SetVar 2, 15, 0
PB.EndWhile
PB.ClearPBMessage
PB.EndOfSub
PB.BeginOfTask 0
PB.ClearPBMessage
PB.SetVar 1, 2, 2 ' RCX ID = 2
PB.SetVar 3, 2, 3 ' Next ID = 3
PB.Loop 2, 10 ' Loop 10 times
PB.GoSub 2 ' WaitMessage
PB.While 0, 2, 3, 0, 1 ' While Message not my RCX ID
PB.GoSub 2 ' WaitMessage
PB.EndWhile '
PB.Wait 4, 100
PB.PlayTone 500, 30
PB.GoSub 1 ' SendMessage (Next ID)
PB.EndLoop ' EndLoop
PB.EndOfTask
End Sub
]]>
If we remove the PlayTone instruction and only waits for 1/100 sec we
have measured the time it takes for 255 rounds in the ring. This
is 74 sec. If we subtract the time for waiting ( 0.01*255*4 sec = 10
sec ) the remaining 64 sec are used for RCX computations and IR
transmission. This is 64/(255*4) sec/ message = 63 msec/message.
We know that the transmission is done asynchronously on a 2400 bit/sec
serial communication channel, and that 1 start bit, 1 parity bit and 1 stop
bit are used per byte. This gives 11 bits/byte and hence 4.5 msec per
byte.
An optimized version of the programs results in a time of 50 sec for
255 rounds. The time per message is then 50 msec/message. With a time
per byte of 4.5 msec we obtain that at most 50/4.5 bytes are sent per
message. i.e. 11 bytes per message. We know that at least one byte is
sent. So either the overhead is large per message or the RCX
compution time cannot be ignored.