For these reasons Y cables used with either keyboard (master) PS2 devices, or with host (slave) PS2 devices are not supported by the standard PS2 protocol and will not work reliably -- and usually not at all.
- PS2 connections can consist of not more than two devices .. one 'master' which is typically a keyboard, and one 'slave' which is typically a host computer, or PS2ADPT-XXX. The keyboard always drives the clock.
- PS2 is a 2-wire open-collector connection. The two signals are designated as the 'clock' and the 'data' signals.
- There is no addressing scheme used since this is a point-to-point protocol. Two keyboards CANNOT be connected to one slave with a Y cable because they will fight for control of the single clock line. Although the clock signal is open-collector, there is no provision for it to be tri-stated, and each keyboard will drive it with its own internal clock. So two connected keyboards would combine their asynchronous clocks on the 'clock' line and corrupt simultaneous keyboard transmissions. And if a slave signaled that it had data to send .. both keyboards would again drive their asynchronous clocks onto the one clock line to retrieve it corrupting the clock which corrupts the slave's data transmission.
- The direction of data transfer is controlled by the slave. If it desires to send data to the keyboard it pulls the data line low during an idle-data period. This signals the keyboard to provide 11 clock pulses to retrieve the data. If two slaves were connected to one master, and one slave requested a transfer, the master would recognize the request and begin clocking in the data. At the same time the second slave would see clocks and data and receive this message as though it came from the master. Although the master would (probably*) receive the correct data .. the 2nd slave would also receive this data which was not intended for it and the consequences of that are are unknown.
*The slave sends a one-bit acknowledge at the end of each character it receives, and with two slaves connected they would 'probably' issue the acknowledge almost simultaneously.