Does anyone make or know of a device to implement "pseudo-half-duplex"
signaling for an async rs232 data stream?

Device would have one input (dataI) and two outputs (dataO and dataP).
Upon seeing a start bit on dataI, device would assert dataP while buffering
a couple mS worth of bits, then begin transmitting the (buffered, delayed)
data on dataO.  Approx .5s after no start bits were seen on dataI, dataP
would be de-asserted.

Ideally, device would be line powered from DTR and bit-rate-independent up
to 19200 or so (9600 might be OK.)  (Hmm.  Does anyone make a line-powered
rs232 driver that has a low-current +5V outout?  That'd be cool.)

Should be a piece of cake to implement.  I don't want to implement it - I
want to point a customer at a low-cost off-the-shelf device...

Thanks
Bill W