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Forum Index : Microcontroller and PC projects : MM2: ONEWIRE question....
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Hello everyone.  I am thinking of using the ONEWIRE protocol to talk between two different MM2 projects. In the manual for the MM2, page 91, it describes how you send and receive data using this protocol and any spare I/O pin. Q1: Does the ONEWIRE "BUS" need to be pulled up idle-high(or low) via something like a 4k7, a-la I2C? Q2: "MM.ONEWIRE returns true if a device was found" - is there any way to make the 'Slave' MM2 device respond to the 'Master' so that the 'Master' knows that the 'Slave' MM2 device is there? ....or is that ONLY valid for true ONEWIRE devices such as 18B20's? EDIT: Q3: Also, what is the effective baud-rate/data-rate/bit-rate of the ONEWIRE protocol used in the MM2 firmware? Edited 2021-02-17 16:11 by Grogster Smoke makes things work. When the smoke gets out, it stops! |
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TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
I wasn't aware that the MM could act as a onewire slave. To answer Q2, onewire does have pullups, usually 4.7k. There is a mode that drives the line high to provide parasitic power and I would not like to see one end driven high and the other end low. Jim VK7JH MMedit |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Hey Jim. No, I don't think it can.(act as a slave) That was one of the things I was asking about. I think I will just use serial instead. I need to squirt it over some 4-core security cable, so am thinking if I use a really slow baud-rate, it might work OK. I will have to do some tests with a length of cable to test reliability. Smoke makes things work. When the smoke gets out, it stops! |
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| matherp Guru Joined: 11/12/2012 Location: United KingdomPosts: 10315 |
RS422? |
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CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
How far? I used twisted pair telephone cable for a 60m run for VCC, GND and serial @9600 without issue, doesn't even seem to care about which wire is twisted with what. Test it on a reel of the cable you intend to use - hopefully you'll be pleasantly surprised. As Peter has already pointed out if you want to do it "properly" then consider 422 drivers at both ends - that also gives you multi-drop so at any point in the future if you want to splice in and add more nodes, no problem. |
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| lizby Guru Joined: 17/05/2016 Location: United StatesPosts: 3378 |
I've successfully run 3.3V serial TTL RS232 over 50 FEET of Cat6 on several occasions (don't remember the baud rate, but I suspect at least 9600, and probably 38400). PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
I agree with Peter.  If you only have 2 cores available for data, RS485 but I like RS422 for full duplex without any need for the extra control lines. Jim VK7JH MMedit |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2642 |
Just tested 40m of 2 pair phone cable (light gauge - 0.2 sq mm conductors) with TTL serial and got reliable 2 way data at 57600 baud. Many errors in one direction at 115200 baud. Pair 1 (blue-white) used for Rx & Tx, Pair 2 (red-black) tied together for ground. CH340 at one end CP2102 at the other. |
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MustardMan Senior Member  Joined: 30/08/2019 Location: AustraliaPosts: 175 |
RS422/RS485 is a differential protocol (2 wires per communications channel). GND is handy to have, but not absolutely essential. With a 6+ core cable (eg: ethernet cable) GND, +V and full duplex communication is easily achievable (in fact, doing it with my current project). Being differential speeds can be significantly higher. I am doing 56kbaud over a meter (for testing) and fully expect that to go at *least* 20 meters. My limits are actually going to be set by the voltage drop on the power conductors and not by the baud rate. Does require a little bit of hardware though. RS232, especially TTL level, is single-ended and pretty good for several tens of meters, but as others have pointed out, distance drops rapidly with higher baud rates. Good for duplex with four cores (with GND and +V), but ethernet cable is cheaper than most other alternatives now anyway. Less than 8 core has become "only if the cable already exists". And for quick and dirty, the wires can be connected directly to the micro with no extra hardware required! Although I'd be expecting to replace my micro on random occasions with such a setup. Cheers, |
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