[b3lha]

I've been talking to Frank, the FreeSSM developer, and he told me about an older version of the SSM protocol. He calls it SSM version 0 (because ours is version 1). The difference is that SSM0 is clocked/synchronous while SSM1 is asynchronous. SSM0 was used on the 3rd Gen Subaru L-Series.

In an asynchronous system, the Select Monitor and the ECU each have a clock. The two clocks run at the same speed (baud rate). Every tick of the clocks, the SSM writes 1 bit of data onto the wire and the ECU reads it.

But on a clocked/synchronous system, there is only one clock, in the Select Monitor. The clock signal is shared between the Select Monitor and the ECU via an extra wire. The Select Monitor writes a bit of data onto the wire and then sends a pulse down the clock wire to tell the ECU to read it.

Anyway, from looking at the 92 select monitor ROM that LAN sent me, I think our Cruise Control Unit uses SSM0 at 250 baud. That's why I haven't been able to talk to it, and why there is an extra wire in pin 4 of the diagnostic plug. When you select "Cruise Control Unit" on the SSM, it switches to synchronous mode, when you exit, it reverts back to asynchronous mode.

With our current interface design and software, there is no way to support SSM0. So there is no way to talk to the cruise control unit. Unless maybe later SVXes had SSM1 cruise control. Anybody know if the cruise control part number changed at any point during production? Maybe the OBD2 cars had an SSM1 CCU?

While it's possible to redesign the interface and software, I'm not sure it's worth the effort. The CCU is probably even dumber than the A/C unit. So I'm going to admit defeat on talking to it for the time being.