METRON QA-ES, User & Service Manual, Page: 31 / 55

(See QA-ES Processor Board Schematic Diagram 1) . All data
transferred between the Sampling Unit and the Processor Unit is
in serial format to simplify the optical interface on the sampling
board. Communication is controlled by the synchronous serial
interface on the processor unit. The signals are transmitted via
pin board base J1.
6. Serial Port
(See QA-ES Processor Board Schematic Diagram 2) . The serial
port is adapted to a 9-pin RS-232C format. The port is set to
9600 baud, 8 data bits, 1 stop bit and no parity. RS-232 driver
U6 drives the data signals. The handshake is software-oriented.
The command responses are returned via the D-sub terminal.
7. Printer Output
(See QA-ES Processor Board Schematic Diagram 2) . The Pro-
cessor Board’s printer output has a standard 25-pin D-sub con-
tact for Centronix interface. The output is built around 3 HC-
MOS circuits; U3, U4 and U5. The circuits are connected to the
data bus and I/O ports of the processor unit. U3 is a latch for the
8 parallel data lines. U4 is the driver for the outgoing com-
mands, while U5 acts as a buffer for incoming commands. RP3
comprises pull-up resistances for the input lines. All signals to
the printer output are filtered to reduce high frequency radia-
tion.
6.2 Sampling Unit
The QA-ES Sampling Unit is placed inside the internal protection
box of the instrument. The card is fixed vertically on the right side
of the protection box. The unit comprises:

power supply;

measuring device, and;

interfaces towards the Processor Board and the Load Board.
The Sampling Unit converts the applied RF signal to a low frequen-
cy signal proportional to the mean square of the RF signal. The peak
value of the applied signal is also measured. The measuring values
are sampled with a 12 bits A/D converter and the data is transmitted
to the Processor Board for calculation and presentation on the dis-
play.
1. Power Supply
(See QA-ES Processor Board Schematic Diagram 1) . The Sam-
pling Unit receives -10 VDC and +10 VDC from the Load
Board via J1). Voltage regulators U17 and U19 provide, respec-
tively, +6V and -6V for the RMS DC converter. Voltage regula-
tors U18 and U20 provide the peak detector with +9V and -9V.
Voltage regulator U16 supplies the opto-coupler in the interface
towards the Processor Board with + 5V.
2. Measuring Device
3