I2C interface to industrial digital I/O boards

This was defined by Opto-22, an American company who specialised in producing opto-isolated industrial I/O.

Between each signal wire is a ground wire to prevent cross-talk.

My only grumble is that the channel numbering starts from pin 47 and works backware to pin 1. If they had made pin 1 as 5V  then the channels would have incremented nicely into wider cables. But it is too late to change now... 

It was such a common standard that Arcom created an adapter board. This routed their four channels to two Opto-22 connectors.

This is the proposed circuit. The two I2C to digital I/O chips are connected in their normal fashion, and the I/O pins connect to the Arcom I/O connector pins coloured in the diagram below. Mostly a join-the-dots exercise.

 I think the only "loose ends" are the interrupt signals, if used.

Most signal conditioner boards will assume they can take a fair amount of current from the +5, +12 and -12 volt pins, but of course the I2C master may not be able to supply that. More thought will be required to solve that issue. 

Should be pretty simple, I2C/SPI wires in and the parallel I/O pins simply routed to a 50-way connector.

Pinout shall follow this standard, with the first device driving groups 0 and 1, the second device driving groups 2 and 3.

I shall omit:

• group 4
• special function pins
• -12V

But I will need:

• +12V if I want to drive my SCB11 relay board
• +5V for most signal conditioner boards

This will be a current supply problem because a USB-to-I2C/SPI board does not supply +12V at all, and the +5V rail does not have a lot of current available - I would guess 100mA or so. The LED32 indicator board will need about 400mA just to power all the LEDs on.

Some external power supply will thus be needed.

The SCB18 provides 32 channels of opto-isolation for DC inputs, with input voltage range to the board -between 3V and 50V - being user-selectable by the inclusion of a series resistor. The board can be configured by switches to be active-high or active-low.

Isolation Voltage		Instantaneous Surge	Working
Input to Output Isolation	700V DC			350V DC
Channel to Channel Isolation	700V DC			350V DC

Each channel has a 270R resistor limiting the LED current, and the phototransistor collectors are pulled up by 4k7 resistor packs:

The SCB11 provides eight channels of relay outputs. Each channel is rated at 240V AC 5A by using high quality relays. Normally-open and normally-closed change-over contacts for each channel are available; on-board LEDs show which relays are active, and links select which I/O lines are used from the 50-way industry standard I/O connector, allowing up to four SCB11s to be 'daisy-chained' on the same ribbon cable.

It has an alternative, much smaller connector which carries the 8 control signals plus 0V and 5V.

It is a very simple board, just a ULN2803 Darlington driver switching some relays.

This will be very useful, showing the signal activity.

The LEDs can be lit when signals are high or low, in groups of eight.

Those chips are 74LS86 XOR gates.

Here are catalogue pages for the some of the many signal conditioning boards that were made.

Arcom originally designed a 50-way connector pinout, but another company made a 60-way superset which they named the Signal Processing bus.

If designing new boards, I would use the 60-way connector because has an even number of pins either side of the board centre line. If using the 50-way connector, place it sits in the subset of the 60-way connector. This will not be symmetrical about the centre line, but that helps keep cabling between the two types free of bends.