Firecracker and USB-Serial

[isw <isw@xxxxxxxxxxx>]

In trying to get this combination to work on my Mac with heyu, I've
discovered that DTR is never going high. With a 'scope, I can see the
"1" signal on RTS pulsing merrily, but the "0" signal on DTR never
moves. Which probably explains why the Firecracker isn't working...

Can anyone shed any light on this problem? I understand that some
USB-Serial adapters won't work with the Firecracker, but I never was
able to find out precisely why (though there seemed to be an assumption
that it had to do with "low signal levels"). That is not the case here
-- the signal on RTS swings a full 5 volts.

Could it be my cheap no-name USB-Serial adapter? Or the open-source
driver I'm using?

Does anybody have a Firecracker working with a USB-Serial? What brand,
and which chip does it use?

thanks

Isaac