Pgm Run (near JP1) allows you to install a header to switch the board between the mode where it
accepts commands from the PC (Pgm) or executes its internal program (Run). Normally you will install
a 3 pin header in this location and a jumper will select between the two options. However, if you are
certain you only want to operate the board in one mode, you can hard wire the jumper.
•
Clock – By default, kits based on the GP3X board use a 20MHz ceramic resonator that includes
capacitors. However, the same socket can accept a regular resonator or a crystal in which case you'll
need to install C7 and C8 with the capacitors required. In addition, an 8 pin DIP oscillator (20MHz)
could be installed at IC4 and its output fed into pin 9 of IC1.
•
Communications – By default, the GP3 communicates via a DCE RS232 port formed by J3, IC2, and
C1-C4. A DCE port mates via a straight cable to DTE ports such as those commonly found on a PC.
However, there are several options available.
First, you can rewire the port so it selectable between DTE and DCE. Near J3 there is a 2x4 pin header
marked DTEDCE. The PC board has traces running across the pins. You can carefully remove the traces
between each of the four pairs of pins using a hobby knife or razor (the traces are on the top side of the
board – the side with the silk screen markings). Then you can install a 2x4 pin header into the holes. If
you place 4 jumpers across the pins (making the same connections as the traces did) the port will be
DCE. However, if you place the jumpers at right angles, the port will be DTE suitable for connection to,
for example, a modem. However, if wired as DTE, you'll need a cross cable (sometimes called a null
modem cable) to connect to a PC for programming. The figure below shows how the DTEDCE jumpers
can be connected.
If you are connecting the GP3 to a non-RS232 device like a microcontroller, you can use the TTLSER
pins and remove IC2. The pins are:
1 – Handshaking in to GP3 (RTS)
2 – Handshaking out to GP3 (CTS)
3 – Transmit from PIC (TX)
4 – Receive from PIC (RX)
-3-
DCE (default)
DTE
DCE (default)
DTE
