Isolated SerialExpress signals

DC-isolated receivers

With copper-based SerialExpress cables, high-speed signals are sent over differential twisted pair 150 ohm conductors between AC-coupled transmitters and receivers, illustrated below. The AC coupling simplifies the receiver/transmitter designs, allowing the bias points of each can be set independently.

Signal integrity is also improved, allowing data to be clocked at higher rates over longer distances. The capacitors have the behavior of open circuits, for low-frequency noise, effectively breaking the ground loop that would otherwise form through the system's green-wire grounds.

DC-isolated transceivers (telecom applications)

In telecommunication communication applications, all cable signals are required to be isolated from ground, which mandates capacitive coupling at the transmitter as well as receiver, as illustrated below. This isn't just a good design practice, in Europe its the law!

Note that consumer and telecom equipment are interoperable, in that consumer equipment can be directly attached to telecom equipment. However, due to the ground-coupling of cable through the consumer device, such mixed systems cannot be used in telecom applications.

Receiver isolation options

Either capacitive or transformer isolation can be used: capacitive isolation is less expensive and transformer isolation is less sensitive to common mode noise. In both cases, termination resistors r_t are used to match the line impedance; resistors r_a and r_b are used to set the receiver circuits DC-bias point.

Note that consumer and telecom equipment are interoperable, in that consumer equipment can be directly attached to telecom equipment. However, due to the ground-coupling of cable through the consumer device, such systems cannot be used in telecom applications.

Isolated cable shields

Cable shields isolate the signal wires from external noise sources and reduce EMI emissions from the signals being transmitted. Although a common shielding technique is to attach the shield to the equipment chassis, this would have several problems, because significant voltages differences may exist between chassis grounds:

• Ground loop currents. Several amps of current may flow through the shields, causing cables to melt and vendors to be sued.
• GFI. Ground fault interrupters may pop.
• Limited applications. Medical and Europen telecom standards outlaw nonisolated signals, which includes the shields.
• Signal integrity and emissions. Problems are hard to diagnose and control.

Consistent with established Ethernet precedence, our cables are fully isolated. The shield isolation is performed at the connector receptable, as schematically illustrated below:

Isolating other cable wiring

The SerialExpress connectors have other signals, which are isolated as follows.

• Power. With a simple cable keying strategy, two not-ground-isolated systems can only be connected by the basic cables, which don't have power conductor wires.
• Identifiers. Cable identifier wires stop in the cable heads, where they are either shorted together or connect to a ground-isolated ROM identifier circuit.

Send mail to dbg@scizzl.com with questions or comments about this web site.
Last modified: May 4, 1997

Isolating other cable wiring

The SerialExpress connectors have other signals, which are isolated as follows.

• Power. With a simple cable keying strategy, two not-ground-isolated systems can only be connected by the basic cables, which don't have power conductor wires.
• Identifiers. Cable identifier wires stop in the cable heads, where they are either shorted together or connect to a ground-isolated ROM identifier circuit.

Send mail to dbg@scizzl.com with questions or comments about this web site.
Last modified: May 4, 1997