PCM-2 Manual.pdf - Voss Associates

Section 13
Modular Detector Board
General Description
External Connections
Each detector in the PCM-2 is served by an independent circuit board which
combines the functions of high voltage power supply, signal amplifier, pulse
height discriminator, and dual channel counter. By mounting these boards
directly to the rear of the detector chambers, the need for high voltage cables and
expensive coax connectors has been eliminated and the effects of any single failure
isolated to one detector.
All functions of these Modular Detector Boards (MDB) are controlled by a
single-chip microprocessor. In addition to the control logic, this device contains
program ROM and working random access memory. The processor, an Intel
83C51FA, is capable of outputting pulse-width modulated signals which are
filtered to provide DC levels which control the high voltage supply and determine
alpha and beta pulse height thresholds. Communications with the main computer
of the PCM-2 are implemented through the processor's internal serial port.
The modular detector board is normally connected to its detector by a banana plug
on the detector body which plugs into a jack mounted on the circuit board.
Grounding is provided via the board's grounding jumper wire/lug which directly
contacts the back of the detector over the MDB mounting stud. When used with a
remote detector such as the PCM-2's optional hand probe, a coaxial cable
assembly is terminated directly to the board.
Low voltage power and serial data enter through a twelve pin header, 11, at the
front edge of the detector board. Pins 1, 2 and 12 of this header are DC ground;
a supply of 8-15 volts DC at approximately 50mA is required on pin 2. As
configured for use in the PCM-2, pins 4 and 5 are bidirectional serial data at
RS-485 levels, buffered to and from the processor chip by IC A5. The remaining
pins of ] 1 are jumpered to ground as necessary to define the network address of
each board. This eliminates the need to manually set address switches when
PCM2.MANI REV AI April 1995 13-1