I apologize for joining this discussion so late in the game, but it appears that the Dallas Semiconductor One-Wire conversation has run its course. So, after reading
@FuzzyWombatSoup 's last post, I thought I would jump in here. Starting over from the original post:
It's been about a decade since I've done much with electronics, but I'm starting to get back into the game. What I'm wanting to do is create an RS485 network with one "master" that sends the data off to an RS232(pc serial port). Each "slave" will have a few sensors that I want to be collecting data(temperature, etc). I'm stuck at an early spot in my design phase and need some guidance
- For each "slave" should I be using a serial output ADC? I'd like one ADC per sensor.
Serial output ADC is a PITA for networked sensors. Better to poll each sensor for data temporarily saved in a register after each conversion.
...
- For each slave, should I be using an RS485 transceiver with more than 1 driver and receiver?
If I have interpreted your last post correctly, you have concluded a μP at each star node is the way to proceed with your project. You need just one bi-directional RS485 transceiver at each node. It remains in "listening" mode until that node is addressed by the "master" at the star junction, presumably a PC of some sort.
- I don't need a microcontroller for the ADC to talk to the RS85 on the slave, do I?
Sure you do. That's why us old farts invented them. Think of modern μPs as "smart" components, on par with resistors, capacitors, etc. but with more than two leads.
- For the master, is it sufficient to collect from the 485 and send it too to an RS232 converter?
The master node should be one or more RS485 COM port(s) on the PC that is running the show. If your PC needs an RS-232 to RS-485 interface there are USB versions available, for example
here and
here. IIRC there used to be PC expansion cards available with RS-485 ports. I like the USB version because it installs as a virtual COM port. If no data is being sent or received it presents a high-impedance "listening state". As soon as you send a byte to the virtual UART it becomes active.
Right now I got a few MAX485, but I'm second guessing whether they are the right tool. As I currently understand it, this is the flow path:
sensor->adc->microcontroller->rs485 transceiver->electrical bus->rs485 transceiver->rs232
I'm looking for the simplest approach with fewest components per node. Even referencing just part numbers will give me na idea of where I need to go
I like the idea of using a MAX485 at each node.along with an Arduino μP for local data acquisition and control. If I were going to make 75 of these nodes, I would probably use a
Texas Instruments MSP430 series μP with built-in ADC. But that's just a personal preference because I happen to have a few avaiable on USB sticks to play with. I would also design a dedicated PCB that is the same for each node, much like
the link in your post.#4 except it would a "smart" node with an RS-485 interface and dip-switch selectable termination resistors.
There is a solution to using RS-485 in a star topology network with 75 nodes. That is way too many nodes for your typical RS-485 transceiver, so make a triumvirate of 25 nodes each. Each remote node CAT5 or CAT6 cable is terminated in a 8P8C modular connector (RJ45) on each end. At the central location of the star "node" each cable plugs into a mating jack that then daisy-chains 25 nodes together on a centrally located jack panel. Since your are using CAT5 or CAT6 cable to each node, and you only require one pair for RS-485 and a second pair for power, a third pair, of the four pairs available, will allow each of the 25 nodes to be daisy-chained back to the star "node" where 24 of the 25 connections there are loop-through to other nodes. The 25th node connects to an RS-485-to-USB converter. You do this three times for 75 nodes. There will be three virtual COM ports communicating with 75 nodes. You can expand this to as many nodes as you desire, providing a different COM port for each group of 25 nodes. Actually, I think you can have up to 32 nodes on each COM port, so the original triumvirate can handle 96 nodes. Depends on the size of the manse I guess.
The daisy-chained RS-485 nodes will need terminating resistors at the far end (the first node of each triumvirate set).