This blog entry I'll take a quick look into the ZX-Key Expansion interface required to get the ZX-Key Keyboard working with a ZX81.
Primarily the expansion board is based on Wilf Rigters' designs. Changes I've made along the way have been minor, and don't on the whole modify how the original design works. Best refer to Wilfs article for an excellent account and explanation. It's enough to say that there are two 74HC245 ICs that handle the decoding of the ZX81s BUS and afford the implementation of a matrix keyboard.
The modifications I've ended up making to the original design have been necessitated by the desire to use the Keyboard component of the project with ZX81s, the RC2014, (or other mircos) and PCs via USB.
For the PCB I added a +5v rail for powering an Arduino Pro Micro which is located on the keyboard PCB and also broke out the ZX81s reset line. In addition diodes used on the row lines have been moved from the Interface to the Keyboard PCB (See the Keyboard Blog Entry - Coming in Part 3).
All seemed to work just fine (after a stupid layout issue and a bit of trace cutting), however on deeper inspections I ran into a major problem when using the popular ZXpand SD card reader in conjunction with the keyboard. The ZXPand would often not read an SD card, or if it did would load corrupted applications or files.
This seemed to be a power related issue, as powering the Arduino Micro via USB instead of through the ZX81 mostly resolved the problem. Removing the Arduino entirely completly solved the issue, with the keyboard working normally. In summary while the decoder interface worked fine, it didn't in combination with the enhanced keyboard.
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| Final Production ZX-Key Expansion Interface |
The ZX-Key Expansion Interface for the ZX81
Primarily the expansion board is based on Wilf Rigters' designs. Changes I've made along the way have been minor, and don't on the whole modify how the original design works. Best refer to Wilfs article for an excellent account and explanation. It's enough to say that there are two 74HC245 ICs that handle the decoding of the ZX81s BUS and afford the implementation of a matrix keyboard.
The modifications I've ended up making to the original design have been necessitated by the desire to use the Keyboard component of the project with ZX81s, the RC2014, (or other mircos) and PCs via USB.
First Iterations
Initially I breadboarded a portion of the circuit for some basic testing, just for a bit of satisfaction. Unsurprisingly that worked perfectly fine, so went ahead and built up a prototype PCB version. |
| Breadboard Test |
All seemed to work just fine (after a stupid layout issue and a bit of trace cutting), however on deeper inspections I ran into a major problem when using the popular ZXpand SD card reader in conjunction with the keyboard. The ZXPand would often not read an SD card, or if it did would load corrupted applications or files.
This seemed to be a power related issue, as powering the Arduino Micro via USB instead of through the ZX81 mostly resolved the problem. Removing the Arduino entirely completly solved the issue, with the keyboard working normally. In summary while the decoder interface worked fine, it didn't in combination with the enhanced keyboard.
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| The First Iteration: Well it seemed to work fine, but a ZXPand looms in the background. |
Second Iterations
To resolve the power problems I added voltage regulator, this takes power from the +9v rail supplying both the interface and the attached keyboard. Additional diodes on the column lines were also added to the keyboard PCB. Thankfully these changes solved all the niggling issues with the ZXpand.
Due to the additional diodes on the Keyboard increasing the voltage drop, the CMOS 74HC245 ICs were replaced with 74HCT245 ICs, the TTL level equivalents. TTL signals provide an extended logic level voltage range counteracting the voltage drop in the signals coming from the Keyboard.
