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Old September 22nd, 2006   #2
Task Group Member
Join Date: Sep 2006
Posts: 26

A fundamental assumption made above is that the transmission medium acts as a wire-and, such that the receiver sees a one if both transmitters transmit a one, and sees a zero otherwise. DMX uses a differential transmission medium, and so a one is transmitted as
D+ = V+
D- = V-
and a zero is transmitted as
D+ = V-
D- = V+
If we assume that the more negative voltage is seen by the receiver -- I'm not entirely positive this assumption is valid, but it seems reasonable -- and transmitter A is transmitting a one, and transmitter B is transmitting a zero, the receiver will see
D+ = V- of B
D- = V- of A
The datasheet for the SN75176 differential transceiver says that if the absolute differential voltage is less than 0.2 V
|D+ - D-| < 0.2 V
the output is indeterminate. In practice, if V- of B is more positive than V- of A, the receiever will see a one, otherwise the receiver will see a zero. The result being that in the case of a collision, the bit the receiver sees depends more on the characteristics of the transmitting transceivers than the bits being transmitted.

Shaun Jackman
Pathway Connectivity
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