CTI - Coaxial Transceiver Interface

no prev chap.  ethernet.html  TP communication
External Transceiver AUI -> BNC

The main component of a MAU for coaxial cable is usually DP8392C or its clone. Let's take a closer look at this device. Four function blocks can be found in its schematic:

  1. Transmitter

  2. Receiver

  3. Collision detector

  4. Transmission timer (limiter).

CTI - Simplified block diagram

  1. Transmitter circuitry converts incoming symmetric signal to asymmetric signal, generated by a modulated current source. When transmission is enabled, this source drives the coax cable with -41 mA current (typical; maximum values are -37 to -45 mA). This DC component is modulated by actual data signal using  +- 28 mA levels. At the same time, edges of the output signal are "rounded" to minimize harmonic noise. Rise/fall times of edges are 25 ns (+- 5 ns)
    Signal output into coax cable
  2. Receiver is a little more complicated. There is a high-impedance, low-capacitance amplifier at its input, followed by an equalizer that compensates uneven frequency response of the coax cable, therefore restoring the rectangular shape of the signal. From the equalizer the signal goes into a noise gate circuit that disables output if the required DC average level is not present on the cable.
  3. Collision detection is based on monitoring average DC level on the coax cable. Input signal, smoothed by a low-pass filter, is lead into two comparators. One of them monitors the presence of DC component and enables transmitter circuitry, as already mentioned. The other one detects a shift in the DC level under about -1.5 V (typ. -1.53 V, max. -1.45 to -1.58 V, see detailed calculations). Assuming 50 Ohm / 2 termination (50 Ohm resistors at both ends), current of 60 mA has to flow through the cable. Output of the collision detecting comparator is converted to AC signal (with a 10 MHz oscillator) and leads out of this circuit to AUI interface. Reaction time (how fast the circuit reacts to a collision) is one of the main delay-influencing parameters that need to be taken into account when designing large-scale networks. Typical delay is 7 to 8 bits (corresponds to 700 to 800 ns).
  4. Transmission timer acts like an "emergency brake" that stops transmission and blocks the transmitter whenever it would transmit for more than 20 ms.

To conclude, coaxial cable is driven by current; however, receivers sense voltage. Therefore, 50 Ohm termination at both ends of the cable is crucial for correct operation. Average DC level on the cable during error-free transmission is around 1 V. Useful data signal has an amplitude of about 1.4 Vpeak-peak.

Electrical isolation circuits usually utilize three miniature transformers in a single DIL16 package. Another necessary component is a voltage converter that isolates MAU circuitry power from other circuits. Usually it is a "black box" in a DIL24 package. For "thin" Ethernet (RG58 coax cable) it must survive 500 V AC for one minute, 2000 V AC for "thick" Ethernet.

Besides these main communication functions, CTI often contains some other diagnostic and testing functions. One of them is SQE/Heartbeat, detailed in TP MAU chapter. Unless specified otherwise, default setting of SQE/Heartbeat is OFF. Using a MAU with SQE test turned on together with a dumb repeater usually causes problems.

References

pdfCentreCOM Micro Transceivers AT-MX10, AT-MX20, AT210 (2 pg.) [local copy]
pdfAN-442 Ethernet/Cheapernet Physical Layer Made Easy with DP8391/92 (10 pg.) [local copy]
pdfAN-620 Interfacing the DP8392 to 93ohm and 75ohm Cable (4 pg.) [local copy]
pdfDP8392C/DP8392C-1 CTI Coaxial Transceiver Interface (12 pg.) [local copy]
pdf16PT-005B YCL LAN isolation transformer (1 pg.) [local copy]
pdfYCL LAN DC/DC Converters (3 pg.) [local copy]
pdfLAN DC/DC Converter YCL Model No. DC-101 (2 pg.) [local copy]
pdfW89C92 Winbond Transceiver for Coaxial Cable (11 pg.) [local copy]
pdfPhilips Semiconductors - AN4004 Electrostatic discharge protection
for the NE83Q92 or NE83C92 Ethernet Transceiver
(5 pg.) [local copy]




Sponzored by LPhard Ltd. Graphics by GIMP Created by EasyPad

(c)Copyright 2000 - 2002, HW server & Radek Benedikt
Web51@HW.cz, Web51.HW.cz
no prev chap.  Obsah  TP communication