User's Manual

3 -Installing a MiY Device  20
Proprietary
3 Secure the device by using the security-screw driver to insert and tighten the security screws
on both sides of the MiY-Device.
Location of Security Screws
4 Verify that the security screws are in place by attempting to rotate the device off the
mounting bracket by its hinge at the bottom. The device should feel secured and not loose.
3.8 Balancing the Termination
For the last device on the line, you should add a termination resister to the terminal block
before plugging it into the device. The following information provides details to assist with the
proper configuration of the RS-485 Standard and is reserved for expert users.
The RS-485 Standard permits a balanced transmission line to be shared in a multidrop mode.
You are allowed up to 32 driver/receiver pairs that can share a multidrop network. The range of
the common mode voltage Vcm that the driver and receiver can tolerate is expanded to +12 to
-7 volts. Since the driver can be disconnected from the line, it must withstand this common
mode voltage range while in the tristate condition. Termination is required for high data rates
and long wiring runs.
Termination is used to match impedance of a node to the impedance of the transmission line
being used. When impedances are mismatched, the transmitted signal is not completely
absorbed by the load and a portion is reflected back into the transmission line. If the source,
transmission line, and load impedance are equal, these reflections are eliminated.
There are disadvantages to termination as well. Termination increases load on the drivers,
increases installation complexity, changes biasing requirements, and makes system
modification more difficult. The decision whether or not to use termination should be based on
the cable length and data rate used by the system.