Using snap modules without the rack

I use your Snap line in our OEM controller, and I came across your Module Integration Guide (form 0876-100610 dated June 2010). Communicating directly with snap modules is something that I am very interested in doing.

According to that document, I can communicate with analog modules using either ARCNET on pins 12(+) and 14(-) or using RS-485 on pins 9(+) and 11(-). For testing, I have a USB-RS485 converted attached to my PC and will communicate with the modules using a application.

Now some questions…

  1. I am not familiar with ARCNET. Does it use the standard RS-485 electrical specs(i.e. around ±5VDC differential pair) or is it something different?

  2. Is there any advantage to using either ARCNET or RS-485?. Would there be a preference to using one over the other?

  3. Do you have any more technical documents regarding communication directly with snap modules? Source code in any language?

As with most things, the first step can be quite large, and I am guessing that once I have taken that first step, I will have more questions.
Thanks for your time

Hi Kevin,

1 & 2 - Arcnet requires a special chip. It really isn’t an option for you because I don’t think any chip manufacturers still make Arcnet chips. So serial is the way to go.

3 - We don’t have any sample code available and as you can see, it’s been a long time since we’ve touched that document. Very few people use the modules directly. I’m sorry we didn’t have the answer you were looking for.

Arcnet came before Ethernet. Originally, most of the network systems out there were all Arcnet and used coax. I think they also had fiber hardware for it as well. As usual, Opto22 was the first one out there using Arcnet on their control systems and they did so because Arcnet is a token ring network meaning that a packet is sent out to the first node on the net from the host, and that host adds to or receives from the packet and sends it on to the next node. If the first node does not answer in a specified amount of time then the next host does the same and so on. This means that you can calcuculate nearly the exact amount of time it will take for a response back to the transmitting host. It’s what is referred to as “determinate network” versus ethernet which is collision based and can vary based on traffic load or other factors. Allen-Bradley attemtped to make a non-deterministic network into one by using software to re-engineer the basic Ethernet design…therefore, it is overly complex and requires special switches.
Opto22 used this early on and was a fantastic system which made the LCM4 with Arcnet a screamer, especially in the days where of 10 meg networks were still the standard and most control systems out there were still running on serial based networks. They therefore incorporated it into their rack cards, making the rack cards scan time deterministic. The whole problem was that the manufactures of Ethernet hardware became dominate which harpooned the whole approach from an external network design. To their credit, Opto stays with what is standard in the industry, except for the attempts of standards being imposed by their direct competitors, such as AB/EnetIP.

Thanks to you both.
I’m glad I’m not missing anything with ARCNET then. I was fairly certain from the get go that I would use the RS-485 connection but just want to ask those in the know.

The main reason for asking for code samples is because I have yet to be able to communicate with modules using the binary protocol. I can comm with it using ASCII at any baud rate, but when I switch to binary I get nothing. I’ll keep trying a bit more and if I can’t get it, I’ll be back with some code that I’ve tried.
Thanks again