Power Devices through Ports or Card Slots

8 Nov 2008

Juniper Systems Field PCs can provide power to peripheral devices through the available ports and card slots. The power supplied through each port or card slot is described below.

Note: Power is not supplied through the COM2 RS-232 serial port on older DOS, Windows CE Field PCs, or the USB Client mini B (ActiveSync) port on newer Windows Mobile Field PCs.


Each port and card slot on the Field PC is designed to supply a maximum current of 500 mA to an attached or inserted peripheral device. The sum total of power being drawn through all ports and card slots simultaneously on a Field PC should not exceed 650 mA of current.

Serial Port (COM1)

The Field PC can supply up to 5 V through the Pin 4 (DTR line) of the COM 1 serial port. This voltage is available continuously to any device plugged into the port such as a barcode scanner or radio transmitter. No DLL or SDK is required to enable this voltage.

Note: Many barcode scanners are built to receive 5 V power continuously through Pin 9 (Ring In (RI) line) of an RS-232 serial port (due to the lack of use of the Ring In signal on desktop computers and other continuously externally powered devices). Since the RS-232 standard did not define which line should be designated for power supply and since the Field PC uses Pin 9 (Ring In (RI) line) for many other power conservative applications, it was a more natural fit to use Pin 4 (DTR line) for power supply. The 9-pin RS-232 serial connector on a barcode scanner may need to be modified to switch from power on Pin 9 to power on Pin 4. Please contact a Repair Center for help determining if your barcode scanner can be modified and to possibly perform the modification.

When the Field PC is turned off, the DTR line supplies 0 V. When the Field PC is turned on, the DTR line supplies either +5 or -5 V, depending on the state of bit 0 in the modem control register, which is controlled by the serial driver. Generally, DTR is at -5 to -6 V (~10 mA) unless the serial port is opened (DTR enabled), at which point the serial driver sets it to the +5 V state.

Note: An inline Schottky (or hot carrier) diode or similar component should be used if a device is not designed to handle the standard negative voltage on the DTR line when the port is not active.

USB Host A Port

The Field PC can supply 5 V through the USB Host A port.

Card Slot (PCMCIA, CF, SDIO)

The PCMCIA card slot on the Allegro Field PC and the CF and SDIO card slots on the Archer Field PC can supply 3.3 V to a peripheral device.

Expansion Pod 50-pin Connector (Allegro only)

The Allegro Field PC can supply 3.3 V through the Expansion Pod 50-pin FFC flex ribbon cable connector (for sealed Allegro Expansion Pod devices only).

All Ports and Card Slots

Caution: It is important to make sure that the power being drawn simultaneously by all attached or inserted peripheral devices does not exceed the 500 mA limit. The 500 mA current limit can be exceeded without risk of damaging the Field PC; however, drawing currents above the limit may cause the battery to become discharged prematurely (sooner than the Field PC`s specified battery life expectancy), causing the unit to perform a low battery shutdown earlier than what may be expected. This is especially noticeable when operating in extremely cold temperatures (below freezing).

Exceeding 700 mA total current may cause the system power supplies to sag, which could force a system reset to occur. Also, short circuit protection can engage at approximately 700 to 1100 mA. Thermal protection engages in the event of a continuous short circuit, which significantly decreases the current until the short is removed.

An external power supply can be attached continuously to the Field PC to counteract some of the negative effects of high current draw from attached or inserted peripheral devices, but this is not a normal use scenario of the Field PC. Specifications for third-party supplies are listed below: