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Difference between revisions of "Talk:DMX512-A"
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(New page: == Timings == The clock rate is 250 kHz so each symbol bit on the wire is 4 microseconds long (period time). Frame rate is ?? Packet rate is typically around 44 Hz when all frames are sen...) |
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| − | + | == connectors == | |
| − | + | The use of modular plugs like 8-position modular connector (8P8C or RJ45) or 6P4C (RJ11)? Is there any allowance for other than 8 position modular connectors? | |
| + | Some product (cheap dimmers) have 6 position connectors (RJ11). | ||
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| + | == Sender/receiver topologies == | ||
| + | To avoid ground loops and improve reception performance, transmitters and/or receivers can be grounded/floating/non-isolated ??? | ||
| + | http://www.usitt.org/standards/DMX512_FAQ.html#FAQ_15 | ||
| + | There are different topologies, and not all will work together. There are no requirement for what topology to use in the DMX specification from 1990, so it was added to DMX512-A. | ||
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| + | Transmitters should use "earth ground" as a reference for the positive/negative voltages that is put on the two data lines. If they don't, it must be clearly marked on the product and in the manual. | ||
| + | Receivers should be ...? | ||
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| − | + | == Voltages == | |
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| + | At http://www.dmx512-online.com/physl.html there is an example wit D1+ at +5V and D1- at -5V, and considering [http://focus.ti.com/lit/an/slla070c/slla070c.pdf page 12 in this document from TI] it is presumably a wrong interpretation. | ||
| − | + | [http://www.dmx512-online.com/physl.html This web page] cites RS485 as having a upper limit of +12/-7 V with respect to ground. This probably refers to common mode loading voltages that receivers must work with. I.e. one of the data wires is allowed to reach +12 or -7 V but the opposite wire must not be at the other extreme, but only differ by 6 V maximum. | |
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| − | http://www. | ||
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| − | + | == Debugging tips == | |
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| + | Links to simple testers? | ||
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| + | How can reverse polarity be detected? | ||
| + | * The break at the very beginning of a packet must be low | ||
| + | * If all transmitted data bytes have value zero, the DMX line should be low about 80 % of the time if there are no extra idle time between slots/frames or packets. | ||
Latest revision as of 13:26, 9 December 2009
connectors
The use of modular plugs like 8-position modular connector (8P8C or RJ45) or 6P4C (RJ11)? Is there any allowance for other than 8 position modular connectors? Some product (cheap dimmers) have 6 position connectors (RJ11).
Sender/receiver topologies
To avoid ground loops and improve reception performance, transmitters and/or receivers can be grounded/floating/non-isolated ??? http://www.usitt.org/standards/DMX512_FAQ.html#FAQ_15 There are different topologies, and not all will work together. There are no requirement for what topology to use in the DMX specification from 1990, so it was added to DMX512-A.
Transmitters should use "earth ground" as a reference for the positive/negative voltages that is put on the two data lines. If they don't, it must be clearly marked on the product and in the manual. Receivers should be ...?
Voltages
At http://www.dmx512-online.com/physl.html there is an example wit D1+ at +5V and D1- at -5V, and considering page 12 in this document from TI it is presumably a wrong interpretation.
This web page cites RS485 as having a upper limit of +12/-7 V with respect to ground. This probably refers to common mode loading voltages that receivers must work with. I.e. one of the data wires is allowed to reach +12 or -7 V but the opposite wire must not be at the other extreme, but only differ by 6 V maximum.
Debugging tips
Links to simple testers?
How can reverse polarity be detected?
- The break at the very beginning of a packet must be low
- If all transmitted data bytes have value zero, the DMX line should be low about 80 % of the time if there are no extra idle time between slots/frames or packets.
