|By Peter Giling|
NXP created several high current output chips for I2C
Unfortunately, these chips are all only available in TSSOP shape,
which makes doityourself very difficult.
Since GCA always have been trying to make all kits really simple to build, this chip was no option this way.
So this was ordered on a small factory made and assembled pc board.
A next nice feature of this chip was the built-in I2C driver, so no extra driver chip was needed, as used on other GCA_PIxx boards.
On board, there is space for two led bars, to indicate the functioning of each ouput.
These leds do not needed installation if you see no purpose there.
The resistor arrays RP1 and RP2 need to be adapted to the voltage as supplied to the GCA_PI08,
so these arrays will be mounted in sockets, making it simple to change if needed.
Also, because of the hight Voltage of 40V max, each port can drive a large serial train of leds, using only one
serial resistor in each chain.
Each port can drive up to 100 mA max.
The GCA_PI08 is part of the RocNet network, Rocrail provides of course all the setting options that are needed.
See: RocNetNode Pi08 LEDs .
The GCA_PI08 has a connection J3
When not used OE+ and OE- should be bridged with a short piece of wire.
The connection of ports can be done in various ways.
First of all there are two PSS 254 connectors, quit familiar in GCA kits.
Instead of that, also a AST 021-10 connector will fit instead of PSS 254.
Here you can simply connect wires without the need of a screw driver.
And all outputs are also available on a 20 pin flatcable header.
This poor quality type of connector is not really favorite inside GCA boards, but is possible anyway.
The standard used PSS 254/PSK254 connectors might be not the first choice of many railroaders, but consider that this connector
can handle a current of at least 1,5 amp att each pin and is very rigid.
All you need to buy is a crimping tools, available from GCA for € 20.
Once you have that settled, you can easily make you own cables in any length with the use of 0,25 mm wire.
The kit will be delivered with standard PSK connectors.
In stead of PSK connectors, as delivered in the kit, also the more costly AST021 connectors can be mounted.
Another, rather expensive option is a 10 pin detachable connector WAGO type MCS. (known for example from Intellibox rail connection)
It will also fit instead of PSS 254 connectors.
The position of the led bars is the next possible option. In this place, a standard 2x8 pins boxheader will fit.
The resistor array keeps its place, so you do not need any addional resistors.
Using boxheaders here also has the nice advantage , that short circuit of any output does not do much harm.
The outside pins of the boxheader are the + for the connected leds, the inside pins are minus.
Please keep in mind to adapt the input power to the amount of leds that is connected to each oupput.
For example do not use 30V power when only one led is connected to each output.
It will result in too much heat in the resistor array!
On version 1.x there is no C3 nor R3.
In the first edition, it was possible to damage the PC9622 chip, when suppply was changed from 15 to 5 Volt.
To avoid that, v2 has an extra capacitor.
If you are using external power to be connected to J3, please be sure that this power is 100% isolated from anything else.
It seems that some power supplies, for example, have their low voltage output connected to ground via the mains sockets.
Where also your PC is grounded that way, serious damage on PI08 pcboard or elsewhere in the RocNet circuit might be initiated.
Be sure of that before you connect it!
There was no reasonable easy and payable way to make a galvanic isolation between RocNet and board PI08.
The first goal in developing this unit was to create something that is not too expensive.
Therefore ground level of RocNet and ground of J3 on PI08 are linked together.
Another issue has occurred with practical test, which never have been realised here in my workshop.
If you connect external power to J3, and later on you deside to switch back to 5V supply for the leds,
it appears that capacitor C2 is fully charged.
Since there is no more drain for that capacitor, it will put the full charge into the 5V circuit,which will destroy the PCA9622 immediately.
Version 2 has a protection for that.
An option for version 1 is to fix a resistor of +/- 10 KOhm between the + and - of the large Elco.
Just fit it at the bottom side of the pcb.
The on-board chip is connected to 5 V of the RocNet bus.
But the common anode of all leds to be connected are seperated from that.
With JP1 it is possible to select a n external supply (coming from J3),
or the 5V from the RocNet bus.
Indication on board shows the right settings for this jumper.
In version 1 it must be absolutely certain that during operation the jumper between pin 2 and 3 may only be connected when capacitor C2 is discharged beforehand.
In version 2 that action is protected.
The PCa9622 is configured to be adressed from &H060 to &H06F.i.e. 16 modules.
Since each chip is controlling 16 Led outputs, the total amount is 256 Led outputs in this RocNet.
This board comes , as from factory, with all adresses left open.
The picture shows the four adresses connected to 0V meaning that this chip is addressed to &H060.
Logic '1' to be selected by connecting the two top pads of the correspondent number, instead of the bottom two.