[Elecraft] Is anyone using a Hardware Data Switch instead of LPBridge?

[Don Wilhelm]

George,

Since RS-232 is a point-to-point protocol, multiple TX drivers cannot 
exist together.
Yes, tri-state RS-232 drivers can be constructed (and were likely used 
in your particular arrangement), but only one driver can be allowed to 
be active at one time.  The fact that you say the drivers (transmitters) 
were "wire or'd" means that the inactive drivers would have to present a 
high impedance.  That is not the normal case for RS-232 drivers.  The 
fact that you state it took 20 man-hours to develop the hardware 
indicates that these are not normal RS-232 drivers.

I repeat - RS-232 is a point to point protocol, and the hardware 
reflects that situation.  One cannot have two active drivers on the same 
line at a time.  The voltages for RS-232 are up to -25 volts for the 
'mark' level, and up to +25 volts for the 'space' level (the idle 
condition or logic zero).  Now think about connecting two active RS-232 
drivers together, one trying to send a logic '1' (-25 volts), while the 
other is idling (at +25 volts).  The combination of the two drivers 
equals zero volts at the receiving end, and zero volts is in the RS-232 
undefined area, so the voltage level should be undefined and ignored.

Further complicating the situation is that some PC serial ports and many 
"almost RS-232" devices have strayed from the normal spec, and will 
respond to zero volts as a logic '1' and any positive voltage as a logic 
'0'.  Normal RS-232 levels would ignore anything between -3 volts and +3 
volts as 'noise'.  But recent psuedo RS-232 receivers will respond to 
that zero volt level as a logic 1 (mark) and receive it as such.

The above situation works for very short 'RS-232' lines as usually 
encountered in PC and ham equipment situations. but will not work well 
with compliant RS-232 receivers that will ignore levels between -3 and 
+3 volts.

In summary, multiple RS-232 drivers on a single line will conflict with 
one another unless special enabling circuits are built into the driver 
and a means of controlling which driver has control of the line are 
present - not a trivial task.  In other words, do not "wire OR" multiple 
RS-232 drivers together unless those drivers are specifically designed 
to allow only one at a time to be active. That means a total system 
control must exist, and that can only happen in specific controlled 
situations.

That is why George's example took 20 hours of hardware design, but 
required 100 hours of writing and debugging software.  This was a 
specific system design and cannot be expanded to "wire or'd" RS-232 
drivers in general - that will not work.

73,
Don W3FPR

On 1/5/2015 10:43 PM, George Danner wrote:
> Harry,
> Years ago we used 8 devices controlled from one PC com port on one RS-232
> data link.
> All of the devices bridged the receive line and were wire or'd for transmit.
> The devices were polled by the PC and only allowed to transmit when polled.
> This worked well with no conflicts for many years.
> So it is possible to use one RS232 port for multiple devices -but- the
> devices must be made to play nice in all circumstances and have special
> hardware interfaces.
> My best memory is we spent about 20 man hours developing the hardware
> solution and another 100 hours writing & debugging software. BTW this was in
> DOS days!
>
>