Serial communication technique

Information can be communicated back from a receiver to a transmitter over a serial link designed for communication to be essentially only in one direction without requiring a separate feedback path with an extra wire. This is achieved by changing at least one characteristic of the receiver input. For example, the receiver input impedance, the receiver input voltage, or both, may be altered. The change in a characteristic of the receiver input can be detected by an appropriately designed transmitter, and such detection can be used to trigger a change in the behavior of the transmitter. Advantageously, only simple alterations are required in conventional serial link circuits to provide a low bandwidth reverse communication path over the same wires. Such a change in a receiver input characteristic can also be employed to inform a transmitter of the existence of a receiver that is on a card that is hot-plugged into system.

TECHNICAL FIELD 
This invention relates to the providing of reverse communication over a 
serial link designed substantially only for communication in one 
direction. 
BACKGROUND OF THE INVENTION 
Many systems employ serial communications. In certain applications the 
serial link is designed for communication to be essentially only in one 
direction. However, at startup, or in the event of an error condition, it 
may be necessary to communicate some information back from the receiver to 
the transmitter. Also, if a card containing a serial receiver is 
hot-plugged into a system, the receiver needs to inform the transmitter of 
its existence. Conventionally, to implement this type of reverse 
communication requires a separate feedback path with an extra wire. If 
such a serial link with reverse communication is implemented for 
communication between integrated circuits, disadvantageously, additional 
pins are required on each of the communicating integrated circuits. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the invention, information can be 
communicated back from a receiver to a transmitter over a serial link 
designed for communication to be essentially only in one direction without 
requiring a separate feedback path with an extra wire. This is achieved by 
changing at least one characteristic of the receiver input. For example, 
the receiver input impedance, the receiver input voltage, or both, may be 
altered. The change in a characteristic of the receiver input can be 
detected by an appropriately designed transmitter, and such detection can 
be used to trigger a change in the behavior of the transmitter. 
Advantageously, only simple alterations are required in conventional 
serial link circuits to provide a low bandwidth reverse communication path 
over the same wires. Such a change in a receiver input characteristic can 
also be employed to inform a transmitter of the existence of a receiver 
that is on a card that is hot-plugged into system.

DETAILED DESCRIPTION 
FIG. 1 shows exemplary serial link 100 arranged in accordance with the 
principles of the invention. The serial link of FIG. 1 includes 
transmitter line driver 101, serial line 105, receiver unit 107, reset 
determining unit 109, reverse signaling unit 111, reverse signal detector 
113, and data transmit unit 115. Serial link 100 is designed for 
essentially unidirectional communication, specifically, only from 
transmitting line driver 101 to receiver unit 107. However, occasionally, 
a signal may be transmitted back from the receiver side of serial link 100 
to the transmitter side of serial link 100 in accordance with the 
principles of the invention. 
Operation of the serial link of FIG. 1 is as follows. Data transmit unit 
115 supplies data to transmitter line driver 101 for transmission to the 
receiver side. Transmitter line driver 101 converts the bits received from 
data transmit unit 115 to line voltages. Transmitter line driver 101 has 
output impedance 103, e.g., 50 ohms, to provide protection of transmitter 
line driver 101 and to provide impedance matching for the termination of 
serial line 105. 
Data transmitted via serial line 105 is received at receiver unit 107, 
where, under most circumstances the data is extracted and the clock is 
recovered. Transmitter line driver 101, output impedance 103, serial line 
105, receiver unit 107 and data transmit unit 115 are known in the art. 
In accordance with the principles of the invention, when a predefined 
condition, such as a) an error occurring in transmission, e.g., loss of 
synchronization, or b) when the receiver is hot-plugged into a system, 
this is detected by, or signaled to, reset determining unit 109. In 
response, reset determining unit 109 signals reverse signaling unit 111 to 
send a signal back to the transmitter by changing at least one 
characteristic of the receiver input, in accordance with the invention. 
For example, the receiver input impedance, the receiver input voltage, or 
both, may be altered. Reset determining unit 109 may also signal receiver 
unit 107 to change its operating mode in anticipation of data transmit 
unit 115 changing its operating mode as its response to receiving the 
reverse signal. 
In the exemplary embodiment of FIG. 1, when signaled to do so by reset 
determining unit 109, reverse signaling unit 111 shorts serial line 105 to 
ground, thus changing both the input impedance and the input voltage. This 
change is detected by reverse signal detector 113, and an indication of 
such detection is supplied to data transmit unit 115. For example, the 
indication may be supplied as a reset signal to data transmit unit 115, 
which causes data transmit unit 115 to reinitialize itself, and, 
optionally, to execute a startup sequence to initialize serial link 100. 
Reverse signal detector 113 may be, for example, a Schmitt trigger. In 
another embodiment of the invention, a comparator may be employed. 
Alternatively, reverse signal detector 113 may also be arranged to detect 
reflections back from transmission line 105, so as to detect a change in 
impedance caused by reverse signaling unit 111. 
The techniques of the invention may be employed for single ended or for 
differential serial links. 
The foregoing merely illustrates the principles of the inventions. It will 
thus be appreciated that those skilled in the art will be able to devise 
various arrangements which, although not explicitly described or shown 
herein, embody the principles of the invention and are included within its 
spirit and scope.