The Mini-Scanner Serial Cable

The Mini-Scanner can be connected to a computer that has a suitable serial port, such as a PC, Mac or PDA.  You can then use the computer to:

You will need a special cable to make this connection.  I can supply such a cable for an IBM-type PC as an extra cost option (about $7) with the Mini-Scanner.  You can also make one up for yourself from the information given later on this page.  For a computer that does not have the same type of serial port connector as a PC (a "DB9"), you must make up your own cable.

Connecting the Optional Serial Cable

To connect the optional serial cable to the Mini-Scanner it is necessary to open the case.  Using a small (#1) Phillips screwdriver, remove the four screws visible from the rear of the unit.  The top of the case, through which the display protrudes, will then lift off, leaving the circuitry of the Mini-Scanner exposed in the bottom half.  Locate the serial port at the bottom right of the printed circuit board.  It is a group of ten gold-plated pins sticking up from the board in two rows of five.  (Don't confuse the DLC adapter port with the serial port.  It looks the same, but is just left of the middle of the bottom edge of the board.)  Place the small ten-way socket that is at one end of the serial cable over these pins and gently press it down all the way.  Make sure all the pins go into holes in the socket.  The ribbon cable should run straight down from the connector with the pin 1 mark (usually a red line) on the right hand edge.  Replace the top of the case and put back the screws (unless you are only connecting the serial port temporarily).  If you did not purchase the serial cable at the same time as your Mini-Scanner, there may be no notch in the top of the case to allow the cable to pass through.  In this case, you will need to make one.

Using an Extension Cable

If you want to use an extension cable between the Mini-Scanner optional serial cable and the computer, you will need a "straight-through" cable designed for this purpose.  It must connect (at least) pins 2, 3, 4 and 5 at one end to the same numbered pins at the other end.  The connector at one end must be male and at the other end female.  This probably seems obvious, but if you pick up the first serial cable you find in a store you might end up with a "null-modem" or "crossover" cable.  This type of cable will not work.  It crosses over the transmit and receive wires so that transmit at one end connects to receive at the other.  This crossing over is done inside the Mini-Scanner and using a cross-over cable will just cross them back again.

You do not need to read further on this page if you have the optional serial cable.

The Mini-Scanner Serial Port Connection

The serial port of the Mini-Scanner is a ten-pin header on the printed circuit board.  An obvious way to connect to this is with a matching header socket, although other ways are possible.  The optional serial cable uses a ten-pin header socket of the "insulation displacement" type.  This assembles quickly and easily onto a 10-way ribbon cable by just inserting the cable and squeezing the header socket closed (in a bench vise, for example).  Tiny sharp forks penetrate the insulation of the ribbon cable and make contact with the wires inside.  If you are making your own cable, you can use other types of connector or even wire-wrap or solder directly to the header.  The table below represents the 10-way header as seen when the Mini-Scanner is held in the normal operating position but with the top cover removed.  The header will be at bottom right.

9 (GND)

7 (CTS)

5 (RXD)

3 (TXD)

1 (NC)

10 (NC)

8 (NC)

6 (NC)

4 (NC)

2 (NC)

The pin numbers shown above follow the conventional numbering of header pins.  This results in the wires in the ribbon cable being numbered consecutively from right to left, just as you would expect.  The marked side of the ribbon cable (pin 1) should be to the right.  TXD is an output from the Mini-Scanner on which it transmits data to a computer.  RXD is an input to the Mini-Scanner on which it receives data.  CTS is an input to the Mini-Scanner which indicates to it whether the computer is ready to receive data.  GND is the common signal ground.  Note that GND will be connected to the vehicle's electrical ground and chassis via a 10 ohm resistor (R24) when the Mini-Scanner is plugged in via the DLC adapter.

It has been pointed out that a conventional serial port connector (a DB9) firmly mounted to the Mini-Scanner case might be better than the header on the PCB.  Certainly, this would be more convenient for connecting and disconnecting the cable and it would also be more rugged.  My reasons for not taking this path were to save cost, to save bulk and to allow the serial cable to be concealed behind the Prius steering column.  The 10-pin header adds very little to the cost or size of the Mini-Scanner so every unit has one whether the user wants the serial cable or not.  Thus the use of a serial cable can be easily added at a later date.  A DB9 would be a different proposition, either adding significant cost to all units or requiring rework if the user decided to use the cable after buying a unit without one.  Rightly or wrongly, I imagine most users will position the Mini-Scanner behind the steering wheel where they can easily glance at the display.  Having a cable run from a bulky DB9 mounted on the outside of the case seemed awkward in this position.  It seemed better to me to run the serial cable behind the steering wheel alongside the adapter cable.  Once behind the dash, it should be possible to run the flat ribbon cable unobtrusively to many convenient locations.

More About the Optional Serial Cable

The cable that I can optionally supply with the Mini-Scanner is a flat ribbon cable and has an insulation displacement DB9 at the computer end.  The flat cable can be passed behind the Prius steering wheel assembly and concealed in the same way as the adapter cable.  You could run it behind the pedals and under the floor mat to the rear seat or pass it over to the passenger side of the car.  The DB9 can plug directly into a "COM" port on an IBM compatible PC or into any computer serial port that uses the same connector and pinout.

Only four pins of the DB9 are used; the remainder are unconnected.  The used pins are 2 (RXD), 3 (TXD), 4 (DTR) and 5(GND).  Since the DB9 has only 9 wires, the tenth wire in the ribbon cable from the 10-pin header on the Mini-Scanner is removed.

Note that pins on a DB9 are numbered along one row and then along the other.  This results in most pins not mating with their like-numbed wire in the ribbon cable and therefore being connected to a different numbered pin on the header at the Mini-Scanner end of the cable.  The following table shows how the relevant pins and signals connect between the two ends of the cable.

Mini-Scanner

 

 

Computer

Signal

Pin

Pin

Signal

TXD

3

2

RXD

RXD

5

3

TXD

CTS

7

4

DTR

GND

9

 

5

GND

TXD

... Transmit Data

(output)

RXD

... Receive Data

(input)

CTS

... Clear To Send

(input)

DTR

... Data Terminal Ready

(output)

GND

... Signal Ground

Signal Grounding Issues

The Mini-Scanner draws its power directly from the vehicle electrical system.  The Mini-Scanner's signal ground is therefore connected to the vehicle's electrical ground and chassis.  This includes the serial port signal ground.  This may create a problem if you connect the serial port to a computer that is also connected to the vehicle's electrical system.  Because of this, the serial port ground connection is via a 10 ohm resistor (R24).  Although this should prevent serious damage, you should verify before connecting up that it is safe to do so by looking for a voltage between the Mini-Scanner's and the computer's serial port GND pins.  It is safe to use a computer that is running from internal batteries or from household power.  It is also safe to use a car power adapter that fully isolates the output from the vehicle electrical system.  However, it may be difficult to determine if this is the case with inexpensive adapters.  Finally, if the car power adapter also makes a connection between the vehicle's electrical ground and the computer's serial port GND pin, this will still be safe.  A problem will arise only if the adapter puts the serial port GND pin at some voltage away from the vehicle's electrical ground.  Current will then flow between the GND pins.  The resistor will limit this current and damage to the vehicle or computer is unlikely, but the resistor itself may be destroyed and the Mini-Scanner may be damaged.


Last edited June 22, 2003.  All material Copyright   2003 Graham Davies.  No liability accepted.