485/DMX Problem at Power Up

[ebrady]

Here is a schematic of a DMX / Power Splitter Board that I made for this years show. With one exception, it works as expected.

A problem is noticed with the following setup PC --> DMX Dongle --> Splitter Board --> DMX 3 Channel RGB Flood from HolidayCoro

When the DMX dongle is actively sending data before power is applied to the splitter board, the 485 transceiver will not properly translate the signal. After that, I can disconnect and reconnect the DMX line and everything works fine. I have isolated the behavior to the SN65LBC169 IC.

I assume I am missing something simple...

So far, I have only had a chance to test it with the 3 Channel DMX module, could this be a behavior related to power on of this module. (Light is being powered from the same power supply which is powering the Splitter).

Thanks in advance.

Schematic.jpg

[David_AVD]

You seem to be missing the required capacitor on the regulator input as well as the common (ground) wire on the DMX input.

[dmcole]

You seem to be missing the required capacitor on the regulator input as well as the common (ground) wire on the DMX input.

Yeah, I'd be really concerned about the 7805 part of the circuit ... typically for a well-regulated supply you'd see a 100uf capacitor on the input and both a 10uf capacitor and a .01uf capacitor on the output side.

I'm not familiar with the SN65LBC179 part and only had time to skim the application note ... my first concern would be that I would think the input to the part would be on Pin 2, not the output. But again, didn't read the spec sheet, so I could be wrong there.

\dmc

[ebrady]

Yeah, I'd be really concerned about the 7805 part of the circuit ... typically for a well-regulated supply you'd see a 100uf capacitor on the input and both a 10uf capacitor and a .01uf capacitor on the output side.

I'm not familiar with the SN65LBC179 part and only had time to skim the application note ... my first concern would be that I would think the input to the part would be on Pin 2, not the output. But again, didn't read the spec sheet, so I could be wrong there.

\dmc

Thanks for the feedback...

I will give the 100uf cap a try, which is the only cap missing that you mentioned. (.1uf is located in the bypass bank). Typically I put the 100uF in, but did not in this case as the power source to this board is being fed from a well regulated 12V/3A power supply which already has quite a bit of capacitance on its output. Still worth a try.

The pins on the schematic are correct for the SN65LBC179, the only time the design has a problem is when a DMX signal is applied before power is applied, otherwise it works fine.

[ebrady]

You seem to be missing the required capacitor on the regulator input as well as the common (ground) wire on the DMX input.

Thanks David.

Historically, I have not run the ground wire, as it as not proven to be an issue with other boards. However I guess there is a good possibility that a ground bias could exist between the nodes of the system as they are both working on different DC power supplies. I will give it a try.

Speaking of bias - I wonder if I should add 4.7K biasing resisitors to the A & B lines on the input.......?

[budude]

The '179 is used in the Renard Simple series with great results - you may want to simply copy the front-end from those designs (the beauty of open HW/SW designs).

[ebrady]

The '179 is used in the Renard Simple series with great results - you may want to simply copy the front-end from those designs (the beauty of open HW/SW designs).

I agree, I have used them on about 4 designs now, not had any problems with them. I checked the Simple Renard 32 board, interestingly enough, I noticed it contains 2- 27K biasing resistors. on the A/B inputs. I am beginning to wonder if this is what needs to be added. Will try tomorrow.


Ed

[David_AVD]

I will give the 100uf cap a try, which is the only cap missing that you mentioned. (.1uf is located in the bypass bank). Typically I put the 100uF in, but did not in this case as the power source to this board is being fed from a well regulated 12V/3A power supply which already has quite a bit of capacitance on its output. Still worth a try.

The input capacitor is there to provide the regulator with a low source impedance. It's not for the normal reservoir function that you were thinking of. On boards where the input of the regulator is more than an inch or two (25mm - 50mm) from the main reservoir capacitor, a local (close to the regulator) bypass capacitor is recommended. The low source impedance is required for regulator stability.

[ebrady]

The input capacitor is there to provide the regulator with a low source impedance. It's not for the normal reservoir function that you were thinking of. On boards where the input of the regulator is more than an inch or two (25mm - 50mm) from the main reservoir capacitor, a local (close to the regulator) bypass capacitor is recommended. The low source impedance is required for regulator stability.

I played with the circuit board tonight and tried the following items with no success

#1 - Added 100uF cap to the input side of the regulator
#2 - Added 485 biasing resistors to A andB
#3 - Tied the DMX dongle and board grounds together.

I am beginning to wonder if this is not an issue with the splitter board, but the 3 channel DMX controller in the RGB flood that I connected for testing. I noticed it is possible to get the 3 channel controller in a strange state by plugging and unplugging he DMX cable a few times in quick succession.

I ran out of time tonight but will connect some of my other DMX boards to this splitter board and see if they exhibit the same behavior.

Ed

[David_AVD]

Might be a case of the cheap 3 channel controller seeing part of the DMX frame and going into channel setting mode?