APC7921

APC7921 overview

The APC7921 is an 8-port Power Distribution Unit where every port is switchable. It provides a way to control the outputs via serial and/or ethernet (10/100Base-TX). There is also a provision to sense and display the total amount of power going through the device via a current-transformer and a 2-digit 7-segment display.

A status-led provides extra device-status, a button allows for basic interaction. A pinhole-reset-button allows resetting the device without cutting power.

The front-panel

Front

• Ethernet w activity-leds
• Serial
• 2x 7-segment display
• 1x 3-color LED
• Button
• factor-reset-switch (hole)
• 8x female outlets
• 8x leds
• 1x male inlet

Schematic overview

                          FRONT
+---------------------------------------------------+-----+
|  Controller | |                                   |     |
+-------------+ |          Relay board              |     |
| IN     OUT    |J131                          J132 |     |
|               +-----------------------------------+     |
|                     +-----------------------------+     |
|                     |     PSU Board               |     |
|                     |J231             J266        |     |
+---------------------+-----------------------------+------
                        BACK

Components overview

PSU module

PSU -> PSU -> controller wiring

The PSU board has two identical PSU's onboard that provide 24VDC The right-most PSU also has a set of pins used for connecting current-transformer circuitry to the controller. The left-most PSU only has 2 pins connected for 24VDC, nothing else.

= PSU -> PSU -> controller wiring

10-wire flat-cable with 2mm pitch 2x5 connectors runs from J266->J231->Controller-IN See connector J266 and J231

Controller

The controller provides ethernet and serial connectivity; as well as a 2-digit display and a 3-color status-led to indicate errors and/or amperage. It also has a single button to interact with the device, as well as a pin-hole switch that allows for factory-resets.

Main things to replicate:

• 24VDC 'merging'
• Faulty PSU detection
• Ethernet function
• Serial function (if desired)
• 24VDC->3.3VDC conversion (for relay-board logic)
• I2C bus to control relay-board
• Adress-bus lines (can be hard-wired)

The controller drives the relay-board with a similar 10-conductor flatcable with 2x5pin 2mm headers. It provides 24VDC and 3.5VDC power to the relay-board with seperate GND's. It also provides an I2C bus (SDA/SCL) and 3 'address-lines'. See connector J131

Relay board

There are 2 main parts to the relay-board. The IO-circuit and the relay-circuit.

Relay-board IO circuit

The relay-board uses a PCF8574 'Remote 8-Bit I/O Expander for I2C Bus' to provide 8 outputs that drive the relays. It is powered with 3.5VDC from the controller-board which creates it from the 24VDC available there. The PCF85674 has 3 address-strapping lines A0,A1,A2 that allow selecting the I2C slave-addresses for both READ or WRITE operations. See table later in the document. In single-board operation, strapping all pins to L would put the READ address at 0x41 and WRITE at 0x40 The relay-boards can be daisy-chained by connecting a next board to the J231 of the first. For this to work, however, at least one, and at most two, address-lines will need to be strapped H The reason for this is the manner in which the A0-A2 lines are presented on the J231 connector. They are shifted one pin,

Signal    J131         +-------------+      J132
  A0        7 ---------|---------+   +------  7   
  A1        9 ---------|--+      +----------  9
  A2       10 ---------+  +----------------- 10

The PCF8574 provides 8 outputs that drive the relays using a circuit that first INVERTS the data via a NOR port (one pin strapped to ground, the other connected to the PCF8574) before running to one of the 8 relay-circuits that switch the 220VAC power provided to one of the 8 output terminals. The 8 NOR ports for this setup are provided by 2x 7402 IC's and seem to function both as a signal-conditioner as well as a way to ensure the power-on state is 'on' for each of the relay circuits (needs to be verified).

Relay-board relay circuit

Each of the 8 relay-circuits is identical and consists of:

• A small switching-transistor driven from a NOR-gate, switching 24VDC
• NMBT222A, marked 7CP
• A Relay-coil being driven by the switching-transistor
• A flyback diode across the relay in reverse-polarity for circuit-protection
• A smaller diode-array that seems involved in driving the LED somehow.
• BAV99, marked A7W
• An LED
• A series resistor to current-limit the LED

Pinouts of connectors

J266 PSU1

(right, next to current-transformers)

                 +----+
              NC |10 9| NC
        GND PSU1 |8  7| Sense PSU1
      24VDC PSU1 |6  5| 24VDC PSU1
Center of 3phase |4  3| Sense Phase A
Center of 3phase |2  1| Sense Phase C
                 +----+

J321 PSU2

(left, on edge of board)

           +----+
24VDC PSU2 |10 9| GND PSU 2
           |8  7|
           |6  5|
           |4  3|
           |2  1|
           +----+

Note: GND PSU1 and GND PSU2 are connected together via screws that fasten PCB to chassis.

J131 Relayboard input

(on relais PCB , left side)

 J131 and J132
  +----+
  |1  2|
  |3  4|
  |5  6| 
  |7  8|
  |9 10|
  +----+

J131 PCF8574 1 GND IO 8 2 GND relais 3 VCC IO 16 4 SDA IO 15 5 24VDC relais 6 24VDC relais 7 A0 IO 1 8 SCL IO 14 9 A1 IO 2 10 A2 IO 3

J132, relay board OUT

On right side of relay board

1 GND IO 2 GND Relais 3 VCC IO 4 SDA IO 5 24VDC RELAIS 6 24VDC RELAIS 7 A2 IO 8 SCL IO 9 A0 IO 10 A1 IO

=== PCF 8574 DW (16 pin) ===

• == pin1 marker.

       +----------+
  A0  -| 1 *   16 | VCC (2.5-6vdc)
  A1  -| 2     15 |- SDA
  A2  -| 3     14 |- SCL
  P0  -| 4     13 |- _INT_
  P1  -| 5     12 |- P7
  P2  -| 6     11 |- P6
  P3  -| 7     10 |- P5
  GND -| 8      9 |- P4
       +----------+