Slave Devices that Require Polling:
This configuration will also work with systems where the devices have to be polled for information. With the setup of Figure 2.3, the switches can be configured so that you still only have to send one set of switching characters. To do this, set the switches up in the following way:
•Set the switch on Level 1 to broadcast to all slaves on startup.
•Set the switch on Level 1 to Auto Select mode, no preamble.
•Set the switch on Level 1 for 0 Port Time
•Set the switch on Level 1 for a short Inactivity Time. Somewhere around 5 milliseconds or so.
•Set the switches on Level 2 for no Auto Select.
•Set the command sequences on the Level 2 switches to be unique from each other. One suggestion is to use a 3 character command. Use the first character as a start of command character like STX. The second character could represent the address of the Level 2 switch you are going to. The third character is the command character.
•Write the software to send the command to select the port. By setting the Layer 1 switch to broadcast and Auto Select, only 1 set of command characters is needed.
Cascading switches becomes more complicated when the devices are a mix of polled and automatic sending of data. It is also difficult to cascade when devices are streaming data constantly, or automatically sending a variable length packet of data.
20 | Documentation Number |
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph
