Comark RF500A/AP Service Manual
Tier 3: RF500A Component Level Repair
Circuit Description
Power Supply
The PSU for RF500A can be broken down into three main power sources each having ancillary electronics associated
with them. The remainder of the PSU is common between these three sources.
External Mains Power
The external mains power is supplied by the in-line power adapter from Friwo (DT12 12V DC, 1A). The support
electronics are simply F311 and D311 providing protection for the circuit.
Power Over Ethernet (PoE)
RF500A is capable of taking power from Ethernet (where supported). U321 converts the Ethernet power into a stable
12V DC, 1A. C321 and C322 provide additional smoothing with D323 providing circuit protection. D321 and D322
provide transient suppression to protect U321 and the rest of the circuit from spikes from the Ethernet lines. R321
provide a minimum dummy load required by PoE switches to be correctly identified as PoE capable.
Battery
The battery pack contains six GP450LAH cells with an internal thermistor to monitor temperature. F301 and D306
provide protection for the circuit. R307 and Q303 provide a method to artificially load the battery to check its current
level of charge.
This pack is charged by U301 and the ancillary components clearly visible on the schematic. The charging current is
420mA, after the pack has been charged for a set amount of time (or has reached a pre-set temperature) charging will
revert to duty cycle charging. Q301 provide control of the charging circuit allowing it to be turned on and off.
Main 5V Rail
The main supply for the PC module is generated by U341 (MAX1685 Switcher), this is controlled via U342 (74HC00)
which is triggered by U421 (MEGA329). The output of U341 is monitored for over-voltage by Q341 and Q342. Once
the 5V rail is present U362 regulates the 3VM rail which is used to power the USB hardware and the RF board.
Permanent 3.3V Rail
U361 is always powered providing a permanent 3.3V rail for U421.
Control MPU Circuit
Main system control is performed by U421, this is always powered and is responsible for monitoring all aspects of the
system and user interaction, the on board ADC is used to monitor most the voltage rails and battery condition. Time
keeping is done via U423 (Maxim DS3231 RTC).
The control MPU provides the PC module with information via a UART to UART connection whilst also serving as a
watchdog for the PC module.
PC Module
The module is basically a credit card sized PC, all the RAM, BIOS, Networking etc are on the board. The only real
connections to the host PCB are the physical connections for the RJ45 connector and power and ground. The main
storage for the module (including the operating system) are the micro SDHC cards located on the host board under the
module. The module has a serial terminal output which is connected to the USB circuit U541(CP2102 UART to USB
bridge) for debug and recovery purposes.
17 November 2011
Page 38 of 65
