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The Maximite Computer, Pt.2

Part 2 - by Geoff Graham

Last month, we introduced our all-new, PIC-32-based microcomputer with its purpose-written MMBASIC language. Now we’re getting on to the fun part: building it!

We mentioned at the end of last month’s article that a kit would be available from Altronics for the Maximite. That has now been firmed up: a complete kit of parts, complete with a pre-programmed PIC-32 and already soldered onto the PCB, should be available this month from Altronics stores, resellers and internet/mail orders for just $79.95 (Cat K-9550). That represents real value for money!

Options

Before you start on construction there are a couple of decisions to make. Firstly, you can set up the video output to be VGA only or composite only or switchable between the two.

The combinations for these options are shown in Table 1. Most constructors will use the VGA output (ie, for a standard computer monitor) and for this option you only need to follow the circuit in Fig.2a and the component list in Table 1 (ie, leave out the 680Ω resistor, leave jumper JP2 out but install a 1N4148 diode in the position marked “D1”).

Click for larger image
Table 1: components used for permanent VGA or composite video output or for the ability to select between the two.

If you want to use composite video (eg, to suit a TV set with a “video in” terminal), you need both the 680Ω and 120Ω resistors, while JP2 can be wired permanently linked. As you will not need the VGA connector, you can leave that out and use the blank section of the rear panel for a panel-mounted RCA connector wired to CON5.

If you wish to switch between VGA and composite you should install a 2-pin header in JP2 so that you can connect a jumper or switch and use the components listed in the last column of Table 1. As explained last month, the video quality is not quite as good as the dedicated modes but it is still acceptable.

Another decision relates to how you plan to use the external input/output pins. As shown in the photographs, we used a right angle IDC header plug. This allows you to connect the Maximite to a variety of external circuits via an IDC socket crimped to a length of 26-way ribbon cable. This is ideal as the connector and cable are cheap and at the other end of the cable you can strip the wires and simply solder them to your test setup.

Another way of using this connector is to use female to male jumper leads to connect the Maximite to a solderless breadboard, as shown last month (page 36). The female end of the jumper wire can be simply pushed onto the pins of the IDC header and the other end plugged into the breadboard. However, be careful which pin is used – they’re easy to mistake.

Instead of using an IDC header plug you could use two rows of vertical header pins. This will enable you to connect (via a matching header socket) to a second PCB mounted in the lid of the case which could hold specialised signal conditioning circuitry suited to your application. There is plenty of room in the case for components on this second board and the completed assembly would make a neat and compact package.

Construction

OK, now we get on with the fun part! If you are building the Maximite from the Altronics kit, it will come with IC1 pre programmed and pre-soldered to the PCB. This makes construction easy as the rest of the components are easy to solder.

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