24-08-2020, 12:50 PM
Hi Ernest,
The Alpha Plus microbee PCB has pads for 10x2 external keyboard connector.
It is designated as X4 and is under the microbee keyboard just above the LOCK key.
This connector presents the 6545 CRTC signal lines MA[4:9], LPEN and a few support signals that are used to drive the keyboard MUX and DEMUX IC3 and IC7 (74xx151 and 74xx156 respectively).
To use an external keyboard, one could use make a little board with their own IC3 and IC7 connected to an external keyboard switch matrix wired up exactly the same as the microbee.
To use a PS/2 or PCAT keyboard, the design must be able to decode the Make and Break events from the (PS/2/AT) keyboard and then translate them into equivalent microbee keyboard events.
Someone uses an inexpensive PSoC 5LP which combines both the programmable logic and microcontroller portions into a single IC.
The design drives the LPEN signal according the the MA[4:9] outputs from the microbee CRTC thus the microbee operates normally none the wiser.
The programmable logic is configured with eight 8 bit registers with each register bit representing a keyboard switch address. 8 registers to represent a keyboard row and 8 bits within a register to represent a column. As the CRTC outputs the refresh addresses, the programmable logic will output the state of the corresponding register bit through the LPEN signal to emulate a keyboard switch closure.
The microcontroller adjusts contents of these registers according to the PS/2 keyboard Make and Break events and is able to support multiple key closures.
This technique also allows a USB keyboard to be used provided that the microcontroller can act as a USB host with its supporting code translating USB HID status reports into microbee keyboard switch events.
Someone is guessing that Microbee Technology's design is similar but using 2 ICs instead of 1 being a microcontroller and separate FPGA/CPLD.
Someone hopes this post is helpful for you.
The Alpha Plus microbee PCB has pads for 10x2 external keyboard connector.
It is designated as X4 and is under the microbee keyboard just above the LOCK key.
This connector presents the 6545 CRTC signal lines MA[4:9], LPEN and a few support signals that are used to drive the keyboard MUX and DEMUX IC3 and IC7 (74xx151 and 74xx156 respectively).
To use an external keyboard, one could use make a little board with their own IC3 and IC7 connected to an external keyboard switch matrix wired up exactly the same as the microbee.
To use a PS/2 or PCAT keyboard, the design must be able to decode the Make and Break events from the (PS/2/AT) keyboard and then translate them into equivalent microbee keyboard events.
Someone uses an inexpensive PSoC 5LP which combines both the programmable logic and microcontroller portions into a single IC.
The design drives the LPEN signal according the the MA[4:9] outputs from the microbee CRTC thus the microbee operates normally none the wiser.
The programmable logic is configured with eight 8 bit registers with each register bit representing a keyboard switch address. 8 registers to represent a keyboard row and 8 bits within a register to represent a column. As the CRTC outputs the refresh addresses, the programmable logic will output the state of the corresponding register bit through the LPEN signal to emulate a keyboard switch closure.
The microcontroller adjusts contents of these registers according to the PS/2 keyboard Make and Break events and is able to support multiple key closures.
This technique also allows a USB keyboard to be used provided that the microcontroller can act as a USB host with its supporting code translating USB HID status reports into microbee keyboard switch events.
Someone is guessing that Microbee Technology's design is similar but using 2 ICs instead of 1 being a microcontroller and separate FPGA/CPLD.
Someone hopes this post is helpful for you.