Microvax connecting from console (MMJ) to DB25(female) DCE port - test-drb@ccmp.vtda.org

15 Jun 2012

Re: Microvax connecting from console (MMJ) to DB25(female) DCE port

...
 > I've even seen one device which powers down
its transmitter unless
> it sees at least one input pin being driven. 
...
  Just to jump in...I don't know about machines,
 but at least one of the later MAX232 family chips can be configured to do this.
 It's a power-saving measure; it shuts down the charge pump.
 Note that this is not the "MAX232", but one of that family,
 which comprises upwards of thirty completely different chips now. 
And someone mentioned another driver chip that can tri-state the drivers.
I once worked in an environment that kinda used that feature
to allow many RS232 devices to share one line.
It was a broadcast-mostly hub-network where clients could
send requests, but they were normally transmit-disabled.
It was collision NON-detect: lack of an ack implied a transmit collision.

Mfgrs such as Black Box also made MSU: Modem Sharing Unit.
The dumb ones just "or-ed" together the inputs.
The "smart" ones used a protocol for
multiplexing many serial ports to one.

I guess EIA-422 or 485 would be better suited for that,
but I suspect the network evolved
...
 from single point to point links to multidrop. 
-- jeffj