Discovery Checksum Problem Analysis - E1.20 RDM (Remote Device Management) Protocol Forums

sjackman · September 22nd, 2006

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

Nigel Worsley · September 24th, 2006

You are forgetting two things:

The SN75176 is a very old device, and more modern transceivers usually guarantee that a 0V input will give a high output. The MAX485 datasheet says: "The receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit."

Also, the controller's terminating network is required to put a slight bias on the line.

Either of these things would ensure that a collision by drivers of exactly equal strength would result in a one being received.

September 22nd, 2006	#1
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

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Incremental Discovery	prwatE120	RDM General Implementation Discussion	1	September 29th, 2021 03:18 PM
Discovery Response Preamble	prwatE120	RDM General Implementation Discussion	0	January 20th, 2007 12:22 AM
Table 6-6 Checksum Example Error	prwatE120	RDM Interpretation Questions	1	January 19th, 2007 06:49 PM
Checksum curiosa	sjackman	RDM General Implementation Discussion	3	October 3rd, 2006 06:24 AM

September 24th, 2006	#2
Nigel Worsley Junior Member Join Date: Jun 2006 Location: London Posts: 13	You are forgetting two things: The SN75176 is a very old device, and more modern transceivers usually guarantee that a 0V input will give a high output. The MAX485 datasheet says: "The receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit." Also, the controller's terminating network is required to put a slight bias on the line. Either of these things would ensure that a collision by drivers of exactly equal strength would result in a one being received.