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Dontronics Home PageDT209 I/O Expansion Board on a SimmStick

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The DT209 I/O expansion board utilizes three I2C bus expansion integrated circuits to provide a total of 24 expansion lines. Each of the three port chips provides 8 i/o lines. These 24 i/o lines allow for input and output expansion without sacrificing any of the general control lines on the SimmBus, i.e. the lines labeled D0 - D15.

Channels: Each color represents the parts accociated with a single channel. Red is channel 1, blue is channel 2, and green is channel 3.

SimmBus Extension: The sixteen lines extend from the SimmBus to the connector row in the same way that the channels do. The coloring shows which of the two groups of lines goes where. Purple/Violet represents D0 to D7 while light blue is D8 to D15.

Each PCF8574 bus expansion device operates by presenting a command byte on the I2C bus and either following the command with data (for output) or reading the data returned by the device (for input). The command byte consists of four bits of device selection (unique to the 8574) followed by three bits that select the address of the individual component. The 8574 has three address lines, and therefore each can be configured with an address that distinguishes the device from others like it sharing the I2C bus. If the three device selection bits in the command byte match the state of the three address lines then the device is selected.

The remaining bit in the command byte determines if the command is intended to be a read or write of the eight data pins. Each of the eight data lines can be independently used as an input of an output. The output lines of the 8574 can even directly drive an LED.

At power-up all lines of the 8574 are high. The part may be used to read from its port pins with a read command and write to the pins with, naturally, a write command. The device may have mixed inputs and outputs, but the pins that are inputs must have previously had a "1" written to them.

Interrupts: Each of the 8574 parts have an open-collector interrupt line that may be tied to three distinct processor interrupt lines, or may be tied together if only a single processor interrupt line is to be used. The graphic above indicates the interrupt line wire points with the color red for channel 1, the color blue for channel 2, and the color green for channel 3.

The 8574 has an open-collector interrupt pin that may be used to interrupt a processor. On the DT209 this pin is connected to a wire point and may be left unconnected or routed as desired with a small guage wire.

Socket Location: Each of the three channels has an I/O socket that may be populated as desired. The socket for channel 1 is red, 2 is blue, and 3 is green.

The design of the DT209 has, if you reference the schematic, a ULN2803 open collector driver associated with each of the three 8574 packages. There is some flexibility associated with the use of this device. Like some of the other SimmSticks, the ULN2803 provides for a high sinking current for driving devices that may need this current capability. Using the DT209 populated with one or more ULN2803s is a good way to interface with relays, small motors, and small lamps.

Resistor Pullups: Each of the three channels has a SIP resistor footprint appropriate for pulling the 8574 port lines either low or high. The SIP package for channel 1 is red, 2 is blue, and 3 is green.

Pullups: The SIP resistor package common pin is connected to a wire point. Channel 1 is red, 2 is blue, and 3 is green.

Resistor SIP pullups will likely be needed for proper operation with the 8574 and ULN2803 combination. Adding pullups to the output of the 8574 will ensure that the minimum specification is met for the ULN2803s.

Power and ground: The DT209 has spare wire points for the connection of power and ground. VDD is shown as red and +12V is shown as green. Ground wire points are blue. 

The 8574 does have reasonable current sink capability, and by removing the ULN2803 and replacing the circuit with shunts or wires, the 8574 can directly drive LEDs. Input can also be applied directly to the 8574 this way.

The 2803 can also be replaced with a resistor DIP package. In this device eight resistors are independent and are connected internally across the package. Use of the resistor DIP package would allow the port lines of the 8574 to be used as inputs, and provide some protection to the device.

Addressing: Each of the 8574 parts have three address lines associated with it. The graphic above indicates the most significant address selection jumper A2 with the color blue, A1 with the color green, and the least significant address selection jumper with the color red.

The DT209 has three address selection headers for each 8574 device. This lets the user select the address of each 8574 to make sure that they are each at a unique address. The headers may be populated for testing or easy address changes, or permanantly wired to a particular address. Pin 1 on the left (square pad) connects to VDD and makes the address a "1", while the other jumper position is ground: "0".

I2C: The I2C bus lines to the three 8574 expansion chips have a place where the connection can be severed (on the PCB). The wire points that are associated with this are above, with SCL in purple/violet and SCK in light blue.

If needed, the PCB traces that tie all three of the 8574 devices to the I2C bus can be cut. This would allow other choices of SimmBus pins to drive the I2C components on the DT209.

The I/O headers that run across the top of the DT209 are a group of two rows of thirty pins above another identical group. These headers provide not only the 24 expansion lines of the three 8574 bus expansion devices, but also the SimmBus signals D0-D15. Each sixty pin header is grouped into five sub-groups of twelve pins (two by six). Each sub-group has eight port lines and four spare lines. The spare lines run to the other header.

This grouping design was done to facilitate connection of the I/O header to external devices. The group of twelve pins allows for a 10 pin ribbon cable header to be connected to each group with provision for two extra signals (such as power and ground). Using 10 pin ribbon headers leaves space between ribbon cable connections. These are the remaining spare pins.

Single connector: All 60 lines can be grabbed with a single connector as shown above.

A single 40 or 50 position cable can also be used for capture of three of the i/o ports. Even a 34 position ribbon cable could be used with the signals that are "missed" being routed back to some of the "spares" by connecting wires on the underside of the DT209

Staggered connections: Five seperate connectors can be staggered on the headers to capture all of the port lines.

Pins can be crimped directly to wires and inserted into a connector housing such as an AMP 9852 or similar and attached to one of the i/o expansion groups.

DT209 Components: (with some optional parts)

This project was presented free of charge to the SimmStick Community to encourage the development of the SimmStick platform. Dontronics has taken the artwork and manufactured the boards so that they are available at the same cost effective price as all other SimmSticks.

This generous work was carried out by the staff of:
J. Gordon Electronic Design, Inc.
201 85th Ave NW
Minneapolis, MN 55433 USA.

And was due mainly to the input and motivation of::
Robert Severson
Senior Design Engineer
Web Page:

Subject:      Re: dt-209
        Date:  Thu, 14 Sep 2000 10:31:41 +1000
              Andrew Riddett <riddy@
Hi Don,
Just purchased a DT-209 and it works great with one small adjustment. Currently VDD (+5V) is connected to pin 11 of the ULN-2803's.  This is fine if you are driving a load that is 5V only however I am driving small 12V lamps.  When the output of the '2803 goes low all is fine, a nice bright lamp,  but when the output is high the internal diode in the '2803 conducts causing the low side of the lamp to be connected to +5V, hence +7V across the lamp and a dimly lit lamp.  I corrected this by cutting the VDD track next to the middle row of resistors on the back of the PCB.  I then connected pin 11 of the '2803's to +12V.
Works great now.  There is a good internal diagram of the ULN-2803 in the back of the Dick Smith catalogue (if you have one) that shows the internal diodes.
Hi Andrew
I didn't even look at this or think of it at the time. Have a look at:
you will see the diodes and the connection for internal and external
VBB. :-(

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