Emulating a PC keyboard

I'd try to get a snapshot of the message with and without and AT keycoard plugged in. When they're plugged in, you can't tell which way the data is going, because it's just open-collector. Maybe just use a couple of pullup resistors, with no keyboard, and see what the laptop sending, then when you plug in the keyboard, see if it's a sort of query/ack - you can spot this if the message is longer than without the keyboard - and if so, then in your device, just respond to the query with the same singal as the ack that you saw.

Hope This Helps! Rich

I have a micro that is emulating an AT keyboard by sending scan codes and simply plugs onto the keyboard port of a PC. This works fine, but if I plug it into a laptop PS2 port it does not work UNLESS I put an AT keyboard in parallel with the data and clock lines. I have scoped the lines and all looks ok and after investigation it would appear that when there is no key activity, there is activity on the clock and data line every second or so which I can only imagine is the laptop asking something from the keyboard and if it doesn't get a reply then it shuts down.

Anyone any idea what this message may be.

Thanks in advance

Iain Wilkie

I think it is just the keyboard saying "Hey, there's still a keyboard plugged in".

Pearce Consulting

First off, just in case anyone is otherwise interested, the keyboard port on a PC has power and two communication lines, CLOCK and DATA. The two communication lines are bidirectional and nominally implemented on the IBM PC side as 7407 outputs with a 10k pull-up and a 47pF to ground, with the output readable by an internal 8042 controller chip. Data exchange is synchronous and can be at any rate, so long as the first rising clock edge starts within a 15ms window after pulling it low to take ownership of the line and so long as it doesn't take more than 2ms to complete an 11-bit dialog, once that first clock starts the cycle. Also, while the clock line is supposed to be high, the keyboard samples it every 60us or sooner to ensure that it remains high. If it goes low, this signals line contention.

There are other timings. 20ms and 25ms for some responses. Etc.

I don't remember reading about anything that operated automatically on a one-second time period, though. But there are messages that could be used by an IBM PC system for some kind of verification of a keyboard connection. In the case of laptops, which have their own built in keyboards, it would make a LOT OF SENSE to be using something like that. You should look at the digital data and see what it suggests.

We could all guess what it is, but there are many possibilities. The BIOS could send something with incorrect parity, for example, and expect an FE response. Or it could be something like Spehro suggests. Or others, such as one of the variety of NOP commands. You will just have to look, I think, or find some site that documents what laptops use. But I think it is possible that different laptop BIOSes may actually use different ideas, depending on the programmer involved (I assume here that there may be at least two widely used laptop BIOS source codes written by different writers.)

Jon

KVM switches have some sort of keyboard and mouse emulation to keep things alive. Might be interesting to watch the KVM interaction. mike

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Hi Iain:

Would you be kind enough to provide me with details (circuit, etc.) on your PC emulator? I will be happy to purchase this info . I have an application I would like to work on. Regards, George

Hi Iain,

did you find out anything, I'm interested too. I think on the notebook, it would be an mechanism of hardware detection, for that your uC must react.

Michael

mike schrieb:

this could be a other way, but currently I have no oscilloscope. But the OP plugs an standard keyboard in parallel and it works, so this should be inspected. KVMs on the other side are very mysterious things: confuse notebooks when connected as PS/2 mouse and keyboard same time, switch some mouses all times to the same pc and so one.

Michael

I think it is a PC __keyboard__ emulator.

Jon

By the way, in case it may help, I have the complete IBM documentation set, "Technical Reference/Personal Computer AT" and "Technical Reference/Options and Adapters/Volumes 1, 2, & 3", which includes rather clear information on the AT keyboard in chapters 1 and 4 in the first mentioned reference. This includes the schematics, as well, plus the complete BIOS listings for the PC side (no 8042 code for the keyboard side that I've found.) If there is something I can look up for you, let me know.

Also, as an interesting aside, there was a feature in the AT BIOS (which, if my talking to BIOS writers at Phoenix some years back is any indication may no longer be supported on newer systems) where you could download code to the AT at power-up through the keyboard port. This was used in motherboard testing fixtures. If you recall, these motherboards did NOT include floppy/hard disk adapters, serial I/O, etc., so this method used as the only standard feature available on every motherboard for which code could be loaded for system testing.

Jon