Keyboard

[stevexyz]

The plus38 board come with the standard ZX81 keyboard connection, but the membrane keyboard is not so common or functional and an alternative might be a good thing.

I've found an old doc written by Wilf Rigter that describe a circuit that will allow a standard "AT keyboard" to be connected to the ZX81 expansion port and properly interpreted on the ZX81. Similar thing can also be implemented with an arduino board (here an example https://hackaday.io/project/166917/logs).

I think that a link to a proper one to use (see for example problems that might arise here: https://hackaday.io/project/166917-tek-v2/log/166879-issues-with-wait-line-on-clones) or a schema of an original implementation might be a nice companionship to this project.

For convenience I copy here the doc text not so visible on the internet, hoping Wilf Rigter likes the spread of the verb :)

ZX-ATKEY - V.2  (c) 2005  wilf rigter
              A ZX81 interface for the AT keyboard

After an 8 year hiatus, I can finally finish the AT keyboard to ZX81 interface.
The PC printer port of the original ROMKEY article has now been replaced
with a PICAXE 18X chip that is programmable in BASIC and has build in
support for the AT keyboard. This is also my first PICAXE project and I am
happy to report that the PICAXE hardware and documentation is easy to use
and the software is easy to program.  The BASIC KEYIN and KEYLED functions
make interfacing the keyboard and decoding the messy protocol reasonably
straight forward.

The 18X ZX-ATKEY design allows use of an AT keyboard as a transparent
substitute for the old ZX81 membrane keyboard. In addition, the PICAXE
has a true asynchronous port that allows the ZX81 to communicate with
a PC at up to 4800 baud. The hardware for this project is amazingly simple
consisting of a PICAXE 18X and a 27C256 EPROM and some passive components.

I build the ZX-ATKEY circuit on a prototype board available from Peter Anderson,
which I modified slightly to mount the PS2 connector next to the DB9 and DC
power supply connectors.

                  THE ZX-ATKEY CIRCUIT

                                    5V                    5V  5V
ZX81 PCB REAR        ZX/ASCII ROM    |                     |   |     AT KBD
EDGE CONNECTOR         27C256       1M        PICAXE18X  4.7K 4.7K    PLUG
                   ______________    | 0.1    _________    |   |
+5V  ---> 5V      |           CE |20-+-||--13|OUT7     |   |   |      DATA
D0   -----------11| D0        A8 |25--------6|OUT0  IN7|16-+---|--------o
D1   -----------12| D1        A9 |24--------7|OUT1  IN6|15_    |   0V-o   o-5V
D2   -----------13| D2       A10 |21--------8|OUT2     |   `---+------o   o
D3   -----------15| D3       A11 |23--------9|OUT3     |            CLOCK
D4   -----------16| D4       A12 |2--------10|OUT4     |
A8   -----------10| A0       A13 |26-------11|OUT5     |
A9   ------------9| A1       A14 |27-------12|OUT6     |
A10  ------------8| A2           |           |         |
A11  ------------7| A3        D5 |17-------17|IN0      |
A12  ------------6| A4        D6 |18-------18|IN1      |
A13  ------------5| A5        D7 |19--------3|IN2      |
A14  ------------4| A6           |           |     SOUT|2--180---- SERIAL OUT    
A15  ------------3| A7           |    5V     |      SIN|3--22K--+- SERIAL IN
A0   -->|--+----22|OE         Vdd|28---|---14|Vdd      |        |
    1N4448 |      |           Vpp|1____|     |    RESET|4 __   10K
RD   -->|--+      |_____Vss______|     |     |___Vss___|    |   |
           |             |14           |          |5        |  0V
IORQ -->|--+---[4.7K]----|            === 0.1     |         |
0V   ____________________|_____________|__________|         |
RST  _______________________________________________________|

                        HOW ZX-ATKEY WORKS

The ZX81 uses the Z80 address lines A8-A15 to scan the membrane
keyboard matrix by pulling each keyboard row low with an address line
and reading the column data through the ULA keyboard port on  D0-D4.
The I/O address for the keyboard port is FEh.

The AT keyboard transmits serial data together with a clock signal to
the PICAXE IN6 and IN7 lines.  This data is sent in the form of a unique
8 bit SCANCODE value together with START, STOP and PARITY bits. The
PICAXE receives the serial data, checks for transmission errors and
translates the AT keyboard SCANCODE into a 7 bit ASCII value which is
output on OUT0-OUT5 together with an active low "keydown" signal on
OUT7.

The ZX/ASCII EPROM converts the ASCII code, which is connected to
EPROM address lines A9-A12 to address a block of 8 bytes containing
the ZX81 scan code. The OUT7 pin is connected via a R/C network to
pulse CE active low for 100ms when a key is pressed after which CE
returns to inactive High. This R/C conditioned pulse is necessary so that
the PICAXE program can quickly return to the KEYIN routine which waits
for the next key code transmitted by the AT keyboard.

The ZX81 addresses the EPROM when the normal ZX keyboard I/O port
FEh is read with an active low on the EPROM OE line and by scanning the
8 EPROM bytes with the A8-A15 lines connected to the EPROM A0-A7 pins.

With CE pin low and OE pin low the EPROM D0-D4 data lines are enabled
and read on the ZX81 data bus in parallel with the ULA keyboard data.

[mahjongg2]

Tricky! Do you know the content of the PICAXE18A chip, without having the firmware you cant build your own ZX-ATKEY Anyway, I have designed my own ZX81 keyboard,, with real buttons (6x6mm size) you can find it in the ZX-81+35 section. I also designed an keyboard overlay, that have holes in the centers of the keys exactly corresponding to the keyboard button-stalks. Also, I have design a similar keyboard but with larger button that have keycaps, for my Rhococo home computer, its keyboard matrix is very similar to the ZX-81. It would be better to start with a simple keyboard, when you know your ZX-81 clone works with that keyboard, you could start experimenting with the PS/2 (AT) keyboard interface.

[mahjongg2]

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