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Theory of Operation
4.1.5 External Remote Serial Interface Selection
An RS-485 or RS-232C communications link on the address decoder/driver module
provides remote serial interface for all switch functions.
The RS-485 interface allows up to 32 devices to operate on a common communication
link. Each device must have a discrete address (there are 255 available discrete
addresses). RS-232C communication is only possible with a single device. Before the
configuration setup, the M&C is jumpered for RS-232C communication, but the switch
will still communicate via RS-485. See Figure 4-1 for locations of the switches and
jumpers.
Use M&C module switch pack SP1 switch 5 (SP1-5) to configure each switch for
external remote interface type, either RS-485 or RS-232C. The ON position (nearer the
PCB) selects RS-232C interface, and OFF selects RS-485.
4.1.6 Battery
A rechargeable battery on the M&C module allows it to retain configuration and status
information for up to 30 days without prime power. M&C module jumper JP6 connects
battery power from the backup RAM. During normal operation, this jumper should be in
the ON position. Refer to Figure 4-1 for jumper location. Should the switch be powered
down, the M&C microcontroller performs the following sequence:
1. Upon power-up, the M&C microcontroller checks the battery-backed RAM for
2. If the battery-backed RAM fails the valid data test, the switch defaults to the
To clear the memory, JP6 must be set OFF, and the leads of capacitor C12 must be
shorted. JP6 must then be set ON again, so the memory can retain a new configuration.
Note: The external battery option (JP8) is not supported in the SMS-758 chassis, and
must be OFF.
4.1.7 Error Response Function
This is a switch function for indicating an error message when a command cannot be
carried out because local processing is using the modem control bus, local keyboard
control, and so forth.
4–4
valid data. If valid data has been retained, the M&C implements the RAM-stored
switch configuration.
bypass mode, and no configuration is loaded.
SMS-758 Modem Protection Switch
MN/SMS758 Rev. 3
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