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Timex Computer 2048

Designed and produced in Portugal. Imported to Poland by Centralna Sk³adnica Harcerska and sold at 106000 ZLP.

The main differences to ZX Spectrum are extra screen modes (512×192 in 2 colors of 8 and 12kB screen with attribbute byte for each data byte separately) and build-in Kempston Joystick interface. Also the keyboard has rubber contats that are better than membrane used in ZX Spectrum.

The main board has factory number 306-950100-VVV, where VVV is version number. Borad versions that I've seen are (ULA date): 03 (8547), 04, 05A (8614), 05c (8627) and 05d (8614). At first sight there's no difference between 05 versions, except the 05A has "IPCB PCP" sign under modulator (as 03), 05c - "SER" under power diodes, while the 05d - "E112E" under the monitor chinch.

The designers made some minor construction errors, that can be corrected by the following upgrades.

/IORQGE correction

In ZX Spectrum the ULA port decoding has OR gate made with resistor (on /IORQ line) and a transistor (on A0 line). As a result ULA is seen in every even port, but an external hardware can take over these ports by pulling up the /IORQGE line with another transistor (like the one on A0).

In Timex Computer 2048 ULA is seen only in #FE port. Other ports used are #F4 (memory switching, not used in TC2048), #FF (screen mode, memory switching and interrupts disable) and Kempston Joystick interface. And the Kempston Joystick interface is the problem here, as is decoded only with line A5=0. So you have to read joystick in ports 0..31, 64..95, 128..160 and 192..224. This gives conflict with many external devices and there's no way to disable the input port without internal changes. (There are also #F5 and #F6 ports decoded for AY-3-8912, but since no device is connected to decoding lines, they don't make any problem.)

Unfortunately, there's no /IORQGE line, like in ZX Spectrum. Well, not exactly. The designers have heard something about this line, but probably didn't know exactly its function. Every main board version has different solution.

In the oldest version 03 this pin is not connected. In version 04b /IORQGE pin is shorted to /IORQ with a jumper. In versions 05 the jumper is removed and both lines are connected together.

The track between /IORQGE pin and /IORQ line can be cuted and separated with a 470ohm resistor - but this only makes the #FE port disableable! The Kempston Joystick still is connected to /IORQ. So second cut is needed and extra wire to reconnect the Kempston Joystick decoder (74LS32, pin 5) to /IORQGE.

Kempston Joystick polarity - TCJOY

The joystick slot has inverted polarity. In normal case the joystick shorts to ground pulled up lines (with 10kohm resistors). In Timex Computer 2048 the input lines are pulled down to ground and joystick shorts them to +5V (protected with 1.5kohm resistor). So most of autofire joystick and Amiga or GEOS mouse will not work correctly.

The resistor R21 must be removed. All 8 10kohm resistors (R5..R12) must be cuted out from ground in 3 places (from R44 towards R39, between R5 and R21 at top side and near to version number at bottom side), then connected to +5V (to the inner pin of R21). And the ground must be provided to pin 7 of the slot - the outer pin of R21 must be shorted to ground pole track between R5 and the slot.

By the way two unused input pins can be connected (pin 5 to bit 5 and pin 9 to bit 7).

Finally, the 74LS244 should be replaced with 74LS240.

2 color LED - TCLED

The Timex Computer 2048 has nice power swith with red LED to show the power on. Two colors LED informs also about the power supply connected and uncharges their capacitors after disconnecting from mains.

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Files for download

tc2048.gif 563.2kB (2006-05-07 19:57)
Schematics of Timex Computer 2048. Very poor quality - GIF version.
tc2048.jpg 966.4kB (2006-05-07 19:58)
Schematics of Timex Computer 2048. Very poor quality - JPG version.
emptyrom.gif 1.9kB (2004-02-05 00:02)
The correct screen when no ROM. On reset, the lines can jump to even or odd columns.

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© 2009-12-28 21:50 Jarek Adamski, http://8bit.yarek.pl