I use both a hardware drum machine (Electron machinedrum) and hardware sequencer (Dark Time).
 
Sonar can be set in preferences to send "play", "song position" and "stop" MIDI commands, but these are sent whenever the Sonar transport is started and stopped.
 
I could really do with being able to send MIDI "play" at a point other than the very beginning of playback (so I can leave blank bars or to control exactly when the hardware kicks in). Anyone know of a way to send MIDI start/stop when I want to using a MIDI event? I've been trawling through the MIDI spec but can't seem to find a way....

Do you mean you want to send a MIDI start signal at some arbitrary point in a project?
 
If that is the case I don't think you can unless it is recognised as a sysx message by the hardware. That will be in your hardware's documentation. You could then insert the message as either Sysx Data or as Sysx Bank (set up in Sonar's sysx view) using the event list view.
 
I've had a quick scan through all of my external hardware's MIDI implementation docs and despite there being loads of messages that can be sent "MIDI start" as a sysx message doesn't appear to be one of them.
 
Somebody may know better than me though in which case I'll happily be corrected.

Midi Time Code maybe?

You can not use Midi start/stop in that way. The way it is intended to work is as follows:

1) from a stopped condition, the master sends the slave the start position pointer which is a beat offset into the song.
2) master sends the continue command and the slave starts playback from the song position pointer.

A Midi Start message automatically causes all slaves to start from zero. The midi sync message has no position information and is used solely for synchronization. In effect, the start and continue messages do nothing on their own and must be followed by a midi clock message. The slaves count the midi clock messages to keep in sync. Slaves will not playback until the first midi sync message after a start or continue.

Sonar does not allow sending midi sync without also sending start, stop and continue.

What you are looking to do can be realized by using a combination of MTC and MMC. Sonar does support this. However very few hardware sequencers have that capability. In this case you would set up an external device with a chase delay, so that playback is delayed by a specific time relative to zero MTC time.
 
Does your hardware sequencer support triggering by midi CC or notes? For example, my Korg Karma WS supports RPPRs which are looped midi sequences triggered by a specific midi note (the midi sequences do not get transposed). The Karma is slaved to Sonar to receive midi sync. I then trigger the RPPR by sending a not on message at the appropriate time and playback is synchronized.

Yeah, I know what MIDI start/stop is meant to do and how it works. I was just wondering if I'd missed something somewhere that would enable me to send starts and stops to hardware using some other method or if the commands could be sent separately from MIDI clock. Obviously, the answer is "no", can't be done that way.
 
However, on digging into the Machinedrum manual, which I've read maybe 100 times, lurking in the global settings it seems there is a way to trigger pattern play and the transort via note on/off after all. Dunno how I missed that, thanks for suggesting the idea. Although given how rigid the Machinedrum clock is it might be an idea to slave the DAW off the MD... :-)
 
The Dark Time however....  Ah well, input monitoring and automated mutes will have to continue to deal with that one.

Hmm. Too bad you can't sync Dark Time to the Machinedrum via the DarkTime's Clk In and St/St In. 
Then you would be golden. :(
 
AAMF, You can try Turning the palyback on/off on the DarkTime's Clk In and St/St In. You'd need a midi clock to analog clock converter and a midi solutions relay which you can program to open/close the relay via a midi message. Just need to supply the correct voltage to the St/St In.
 
When all else fails, hack away. :D
 
 

Yes, MIDI to analog/cv would be one way to do it. More complexity and expense though....
 
Not helped by me being a musician, not an electronic engineer :-)