PS/2-to-USB adapter - why does this work?

[George]

I see this for about $6 at Newegg:

http://www.byteccusa.com/product/adapter/bt-2000.htm

It lets you connect both a PS/2 keyboard and mouse to one USB socket
on your computer (for laptops which have no PS/2 ports).

Just looking at it, and with the price being so low, it appears that
this may just be straight-through wiring, with no processing or
logic circuitry at all.

If that's true, then I'm confused. I'm no expert on either the PS/2
or USB protocols, but I just always thought they were not
compatible, as well as being pretty complicated (well, USB, at
least). Particularly if, as is the case with this adapter, you
parallel two separate devices at the input. I've checked on my old
desktop, and the two PS/2 ports are not paralleled - only the power
and ground pins are common.

So I guess I'm saying that I'm surprised this works. Does anyone
have an explanation, or even a link to an explanation?

I just got a new laptop, and just assumed I would need to buy new
USB keyboard and mouse stuff for those times when I just need to go
faster than the laptop's touchpad and keyboard will permit. But
apparently not. And I really really really like my old keyboard and
mouse. So this is officially nifty if it works.

 

[Michael Black]

I see this for about $6 at Newegg:

http://www.byteccusa.com/product/adapter/bt-2000.htm

It lets you connect both a PS/2 keyboard and mouse to one USB socket
on your computer (for laptops which have no PS/2 ports).

Just looking at it, and with the price being so low, it appears that
this may just be straight-through wiring, with no processing or
logic circuitry at all.

If that's true, then I'm confused. I'm no expert on either the PS/2
or USB protocols, but I just always thought they were not
compatible, as well as being pretty complicated (well, USB, at
least). Particularly if, as is the case with this adapter, you
parallel two separate devices at the input. I've checked on my old
desktop, and the two PS/2 ports are not paralleled - only the power
and ground pins are common.

So I guess I'm saying that I'm surprised this works. Does anyone
have an explanation, or even a link to an explanation?

I just got a new laptop, and just assumed I would need to buy new
USB keyboard and mouse stuff for those times when I just need to go
faster than the laptop's touchpad and keyboard will permit. But
apparently not. And I really really really like my old keyboard and
mouse. So this is officially nifty if it works.

This is hardly a design question, but you forget that design isn't just about
making something, but often about making something cheap.

When something sells well, the manufacturer can lessen their cost
by buying in quantity and going to custom components that result
in lower manufacturing cost. They can then translate that to a lower
price to the consumer.

That lower price increases demand for the item, which can in turn cause
another iteration in design that drops manufacturing costs again, which
drops the selling price further.

The history of electronics has always been this history. VCRs were out
of sight price wise when they first went on the market, and they were
made with many off the shelf components and as a result were big and bulky.
But as consumer demand increased, manufacturers were able to use higher
integration components, which brought prices down, and so on. The
price drop seems even more spectacular with DVD players.

So your equation is missing this. You don't need to seek some false
path to understanding of what's going on in that device, you simply need
to know that what's inside has now been compressed down to what amounts
to a blob of epoxy over a bit of silicon and the cost of designing
such high integration is spread over a large number of the item being
sold.

Now, if this had been soemthing that few people would be interested in,
a low price might indicate something that is making do with some wires,
because demand could not reflect a low price.

Mice and keyboards aren't particularly simple devices, floppy drives
either, yet all can be had for a song now, while they cost hundreds
of dollars a couple of decades ago.

Michael

 

[AZ Nomad]

I see this for about $6 at Newegg:

It lets you connect both a PS/2 keyboard and mouse to one USB socket
on your computer (for laptops which have no PS/2 ports).

Just looking at it, and with the price being so low, it appears that
this may just be straight-through wiring, with no processing or
logic circuitry at all.

If that's true, then I'm confused. I'm no expert on either the PS/2
or USB protocols, but I just always thought they were not
compatible, as well as being pretty complicated (well, USB, at

That adapter has a USB interface to two serial ports most likely a single
chip embedded in the USB plug. When connected, the PC sees the usb
device, reads it's registers to discover two HID (human interface device)
ports.

 

[Joel Kolstad]

I see this for about $6 at Newegg:
http://www.byteccusa.com/product/adapter/bt-2000.htm
It lets you connect both a PS/2 keyboard and mouse to one USB socket
on your computer (for laptops which have no PS/2 ports).

Just looking at it, and with the price being so low, it appears that
this may just be straight-through wiring, with no processing or
logic circuitry at all.

Nope, it does have circuitry inside of it (built into the USB connector under
the "overmold")... probably a single IC that's a microcontroller with --
possibly -- some USB-specific hardware built-in (you can actually bit-bang
low-speed USB at 1.5Mbps, which works fine for keyboards/mice, and this is
almost always cheaper than buying a microcontroller with full speed USB
hardware included).

They're probably making them 10,000-100,000 at a time and thus the
microcontroller price is going to be down around a dime or maybe a quarter...

 

[Jim Thompson]

Nope, it does have circuitry inside of it (built into the USB connector under
the "overmold")... probably a single IC that's a microcontroller with --
possibly -- some USB-specific hardware built-in (you can actually bit-bang
low-speed USB at 1.5Mbps, which works fine for keyboards/mice, and this is
almost always cheaper than buying a microcontroller with full speed USB
hardware included).

They're probably making them 10,000-100,000 at a time and thus the
microcontroller price is going to be down around a dime or maybe a quarter...

I had one of those prior to going to a switch box. IIRC it IS
straight thru... mouse and keyboard use the same connector style, but
different pins.

...Jim Thompson

 

[Joel Kolstad]

I had one of those prior to going to a switch box. IIRC it IS
straight thru... mouse and keyboard use the same connector style, but
different pins.

Weeellll.... there are plenty of keyboards (and mice) that can "speak" both
USB and PS/2 and use "straight thru" adapters, but the link the OP provided is
specifically for PS/2-only keyboards and mice... the PS/2 protocol is just a
variant of I2C, whereas USB is quite sophisticated.

 

[AZ Nomad]

I had one of those prior to going to a switch box. IIRC it IS
straight thru... mouse and keyboard use the same connector style, but
different pins.

Only if the keyboard to be used has a USB controller and switches modes
when connected to the PS/2<>USB adapter. Mice do this all the time.
However, the trick *only* works with an adapter shipped with a particular
keyboard or mouse. They don't do protocol conversion; they signal a
particular keyboard/mouse to perform the USB protocol over the PS/2 pins.

 

[jasen]

I see this for about $6 at Newegg:

Just looking at it, and with the price being so low, it appears that
this may just be straight-through wiring, with no processing or
logic circuitry at all.

impossible. straight through ones should be half that price anyway.

So I guess I'm saying that I'm surprised this works. Does anyone
have an explanation, or even a link to an explanation?

it's probably got a $1 microcontroller in it doing the conversion.

Bye.
Jasen

 

[jasen]

On Wed, 3 Jan 2007 10:17:13 -0800, "Joel Kolstad"

mouse and keyboard use the same connector style, but different pins.

nope, they use the same pins, the only reason why they aren't usually
interchangable is software. linux (>2.6) doesnt have that limitation.

Bye.
Jasen