View Single Post
 September 22nd, 2006 #2 sjackman 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 Code: ```D+ = V+ D- = V-``` and a zero is transmitted as Code: ```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 Code: ```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 Code: `|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. Cheers, Shaun Jackman Pathway Connectivity