Frequently Asked Questions

General Hardware

• What is the pin-out for the RS-232 port on the HC/HX-20 units?
• How do I open the case of the hx-20? Which screws should I pull out and which ones is it best to leave in?

Batteries and Power Supply

• How often should I replace the battery pack?
• What type of batteries does the HX-20 use?
• How do I change the battery pack?
• How is the power supply configured? Does the HX-20 charge its the batteries internally, or does the power supply do this?

General Hardware

What is the pin-out for the RS-232 port on the HC/HX-20 units?

The pin-outs for the 8-pin port are as follows:

The 5 pin port is similar, but pins 1 to 5 and Frame only, of course :) Both ports are DTE wired. Typical voltage is +/-8V max. The 8-pin port is 110-4800 baud, 9600 transmit-only. 5 pin is normally left at 38400, but can do 3 other rates as well. You need to use assembler to go reliably above 300 baud for receive on the main 8 pin port (BASIC isn't fast enough).

How do I open the case of the hx-20? Which screws should I pull out and which ones is it best to leave in?

1. Undo all the screws on the bottom.
2. Turn HX-20 over onto a soft cloth (holding the top and bottom together by hand).
3. Gently lift the top up at the back about 1 to 2 inches (2 to 5 cm). You will see a plastic cable going from the bottom board to the top.
4. Gently lift the locking latch up on the connector, and then slide the cables up and out. At this point, the top is connected only by the front keyboard cable - ignore this.
5. Place another soft cloth on the table at the front, and place the top upside down in front of the bottom.
6. If you are replacing the battery, you can now access the battery pack:
   1. The cable is held by a locking latch - unlock this, and remove the battery pack screw.
   2. Remove the metal plate, and lift the old pack out. Use the cable and connector to wire up a new battery pack.
7. Reverse the process to re-assemble (remember to lock the LCD cable).

Batteries and Power Supply

How often should I replace the battery pack?

The usual advice is to replace NiCd packs every 10 years or so, or more frequently. So if you have the original ones, they are definitely in need of a change. Old NiCd batteries can do nasty things to electronics as they can produce a corrosive gas when they leak with age. Bear in mind that both the EU and the USA are phasing out the use of NiCd batteries, so in a few years time replacements will be unobtainable.

What type of batteries does the HX-20 use?

The HX-20 uses four sub-C cells soldered into a 2x2 arrangement.

How do I change the battery pack?

1. Open the HX-20's case following these instructions.
2. Wire up a new pack using the old cable and connector. The battery is 6V @ 2000mAH. You need 4 sub-C cells. The batteries are wired in series.
3. Charge the new cells for about 12 hours for the first time before use, then 3 further charges of 10 hours after using the HX-20 to the charge battery prompt each time.

How is the power supply configured? Does the HX-20 charge its the batteries internally, or does the power supply do this?

The power supply is a simple 6V@600mA wall brick. The charging circuit is totally dumb, and will cook batteries if left connected for too long. NEVER use the HX-20 without batteries as this will damage the internal circuitry.

Copyright © Julian Wald 1997-2004
Most recent revision September 2004