Auxiliary Output; Interface Pcb; Servicing Note - TTI QL355 Service Manual

Auxiliary Output

Each transformer has a 4Vrms winding and these are connected in series at the Auxiliary Power
pcb. IC3 is an adjustable low drop out regulator. The 3 preset output voltages are achieved by
changing the value of R7, by switching resistors in parallel with it. The regulator is turned on by
pulling pin 2 low. IC3 has internal current limiting and when the limit is reached the output voltage
will start to fall; this is detected by comparator IC5, by comparing against the 2.5V reference IC4,
turning on Q1 which turns on the I-limit led and the opto to signal current limit, which is used only
by the remote interface.

Interface Pcb

The GPIB board is controlled by the Z80 based processor in IC6, which has associated EPROM
IC7 and RAM IC8. IC6 contains two UARTs. UART 0 generates and receives data lines CTRL TX
and CTRL RX which are used to control the power supply hardware. These signals are sent and
received over an RS485 type bus which communicates with the PIC processor(s) residing on the
control board(s). This bus is opto-isolated on the control boards. The internal control bus driver
consists of IC3-D and Q1. The receiver consists of IC3-F and IC3-E.
The second UART in IC6 is used to send/receive over the external RS232 and USB buses. UART
1 I/O signals, COMMS RX and COMMS TX, are selected in IC16-A and IC16-B. When RS232 is
selected as the power supply's communication bus (USB SEL low), these gates switch COMMS
RX and COMMS TX to RS232RX and RS232TX. The RS232 interface conforms to the
proprietary ARC standard. The drivers and receivers for the RS232 interface consist of IC1 and
IC2. When USB is selected as the communication bus COMMS RX and COMMS TX are
connected to USB RX and USB TX . USB RX is generated by IC17 and USB TX is routed to
IC17.
IC17 translates USB signals to RS232 type signals. It therefore only works when the USB
interface is driven by special USB drivers in the communicating controller (e.g. PC) which encode
RS232 signals onto the USB. IC17 is accompanied by IC15 which is an EEPROM programmed
with product specific data. The controller reads this data when the power supply is connected to
the controller's USB port. The data is used in the automatic 'enumeration' process within the
controller.
IC16-C, R9 and Q2 are used to switch R15 onto the USB data + line. When R15 is connected to
+3.3V, generated by IC17, it identifies the device to the controller as a full speed USB device. It is
disconnected from +3.3V if USB is not selected as the power supply's communication interface
(USB SEL low) or if there is no voltage present on the USB power line, VBUS.
IC9, IC10 and IC11 form the GPIB interface. IC9 is a GPIB controller IC and performs all the
GPIB hardware handshaking and status reporting. IC10 and IC11 are GPIB specific buffers. The
processor ignores the GPIB interface if GPIB is not selected as the power supply's
communication interface. Conversely it ignores COMMS RX and COMMS TX signals if GPIB is
selected as the communication interface.

Servicing Note

If IC31 the memory IC located on the power pcb is replaced it is very likely the check sums do not
match resulting in "error 999" being displayed. Pressing OK will get out of this condition and
initialise the EEPROM. This will only work on a single power supply or on output 2 on a triple
power supply. If IC31 has to be replaced on output 1 side of a triple power supply, output 1 and
output 2 power boards will have to be interchanged, also links will have to be changed (see table
under Power Pcb desciption) so that the power supply can initialise.
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