Lexicon
7-49
Option Board Interface (sheet 4)
Connector J6 brings out signals to support an additional plug-in option board for additional I/O and memory.
PCMCIA Interface
The SA-1100 provides control signals to support a two-slot PCMCIA interface. Slot 1 is used for the on-
board program Flash memory and slot 2 is used for the option board PCMCIA card slot.
Expansion Serial Interfaces
Serial Port 0
Serial port 0 is a universal serial bus (USB) device controller that supports three endpoints and can operate
half-duplex at a baud rate of 12Mbps (slave only, not a host or hub controller). This port is connected to the
option board connector for future expansion.
Serial Port 2
Serial port 2 is an infrared communications port (ICP). It operates at half-duplex and provides direct
connection to commercially available Infrared Data Association (IrDA) compliant LED transceivers. This port
is connected to the option board connector for future expansion.
Debug Interface
The debug Interface available through the option board connector uses Serial Port 3 of the SA-1100. This
port is a combination synchronous data link controller (SDLC) and universal asynchronous
receiver/transmitter (UART) serial controller. For LARC2, it is configured as a UART.
Power Supply
Power Source (sheet 7)
The LARC2 operates from a source of 10 to 12VDC, which may either come from the host 960L via the 9-
pin HOST connector J20 or from a dc source connected to the EXT. POWER jack J21. When connected to
the regulated 12VDC supply in the 960L, the LARC2 voltage at J20 will be lower due to IR losses in the 9-
pin cable, and this limits the length of cable that can successfully supply the operating current of 1-to-2
Amps. A plug in J21 opens the switching contacts A&B, which disconnects the +12HOST and substitutes
the external supply. External power on the center pin of J20 (pin D) connects through FB3 and D4. D4
prevents damage due to a misconnected external power jack (AC or the wrong polarity of DC).
Main 12V (sheet 7)
The voltage at the cathode of D4 connects through PS1 to become the main dc supply, nominally +12V,
reduced by the diode drop. PS1 is a 0.75 Amp self-resetting thermally activated fuse. Normally, the fuse
exhibits a low series resistance, a few tenths of an ohm. When overloaded by currents >1.5Amp, the fuse
undergoes self-heating and switches to a high resistance state due to the thermal characteristics of its
material. This high resistance limits the current drawn from the supply under the overload condition. The
fuse maintains the high-resistance state as long as it dissipates about 0.8W, or about 66mA at 12V, which
is the short-circuit condition. The trip time is typically 0.2 seconds at 8 Amps, which is a severe overload.
Smaller overloads can take many seconds to trip. Resetting occurs when the load is removed and the fuse
cools, returning to the low-resistance state. When PS1 is tripped, the voltage across it is sufficient to supply
current through R16 to illuminate LED D3. D3 glows as an indication that the fuse has tripped, visible
through the ventilation slits in the bottom rear cover.
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