# Bus topology question

1. Oct 3, 2009

### Math Is Hard

Staff Emeritus
http://en.kioskea.net/contents/initiation/topologi.php3

What connects the cables coming from the computers to the "bus"? I can't quite get the idea of connecting these cables to another cable when connecting devices aren't mentioned.

2. Oct 3, 2009

### mgb_phys

The bus is just the cable running between each computer.
Like an ethernet cable - it's called a bus when it's shared (like the original ethernet) it would be better drawn with the wire looping form one machine to the nex

3. Oct 3, 2009

### Math Is Hard

Staff Emeritus
Thanks, MGB. So would it actually look more like this if it was physically set up?

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4. Oct 4, 2009

### mgb_phys

Ironically it would be better to draw it as a star. The point of a bus is that a voltage on it is immediately seen by all interfaces connected to the bus.

(A star network is different - there is usually some logic in the hub to control where the data goes)

5. Oct 4, 2009

### Math Is Hard

Staff Emeritus
hmm.. I'm having trouble drawing it as a star configuration without a hub (or switch) since, as you said, then I have a star topology. I less trouble envisioning it wirelessly, but I know that these networks were originally set up using coaxial cable, so there must be a way to diagram that design.
Do you mean a switch? From what I've been reading, hubs seem pretty dumb, just sending the data on to every other host on the network. But I think you could have either a hub or a switch to connect the star network (I don't know this for sure).

6. Oct 5, 2009

### Math Is Hard

Staff Emeritus
I found the solution. The cables can be connected to other cables with T-connectors.

7. Oct 5, 2009

### Staff: Mentor

The two choices for connecting your devices to a multi-drop doubly-terminated bus are Daisy-Chain or with stubs.

For a given network type, there will be a specification for how long the stubs can be. For RS-485, for example, zero-length stubs (Daisy-Chain routing of the network twisted pair) are mandatory. For some networks, like the 78kbps control network that my company makes transceivers for, the stubs are allowed to be up to 3 meters long.

8. Oct 5, 2009

### Math Is Hard

Staff Emeritus
Thanks, Berkeman. What is meant by "multi-drop" and what are "stubs"?

9. Oct 6, 2009

### Staff: Mentor

Your cat is scaring me. But I'll try to answer under duress... :tongue2:

Multi-drop means more than one TX node and one RX node. Two nodes is point-to-point at the ends of the DT bus, which is regular Ethernet comm. >Two nodes is multidrop.

But in control networks and other specialized networks, you want up to 64 nodes all working together, and that's when the multidrop 64 node network comes into play.

10. Oct 6, 2009

### Staff: Mentor

And sorry. Stubs are the short lengths of twised pair cables that you use to connect the main bus TP to the nodes. They need to be electrically short,

11. Oct 6, 2009

### Math Is Hard

Staff Emeritus
Thanks mucho. I'll change my avatar later. Just for you.

12. Dec 15, 2009

### mugaliens

Bingo, and you got it right in your first diagram.

Called 10Base-2, or thinnet, the cabling was coax, and it ran a straight line, with terminators at either end. Tapping into the cable via T-connectors were leads which went to the coax cards in every computer.

That was truely the last true "bus" design in the business, as all others offer some sort of hub and spoke design, with 10Base-T being the first, basic hub, rebroadcasting data arriving at one port to all ports.

A switch simply applies some intelligence, figuring out from the packet whether to send it to a specifically intended receiver, to that receiver, or whether to rever to hub mode and broadcast. Layer 3 switches went a step further, but that's getting beyond the scope of the question!

Yes - both hubs and switches are star topology networks.