Tait TB9100, Service Manual, Page: 80 / 132

80 Network Circuitry TB9100 Reciter Service Manual
© Tait Electronics Limited January 2006
the base station supplies if the isolation voltage requirements for the audio
interface are to be maintained.
M-Lead Interface
The M-lead is used for outgoing signalling and is controlled by an opto-
MOS relay DS500a; this device has a pair of MOSFETs connected to form
a switch and is driven by a photovoltaic diode illuminated by an LED. When
on, the switch is capable of conducting in either direction. The input LED
is driven by emitter follower Q501a. This emitter follower boosts the output
current drive of the MPC output pin, PB21.
Overvoltage protection for the switch is provided by D501, which is
followed by the usual EMC filtering. Protection device D501 has an SCR
type structure so it latches on in response to a transient. In the event that a
low resistance DC source is connected to the M-lead, a prolonged excess
current could eventually damage the EMC inductors or the PCB tracking.
Fuse F501, in series with the M-lead, protects against this eventuality.
E-Lead Interface
When E-lead current flows through the input LED of opto-isolator
DS500b, it causes the opto-transistor to conduct pulling input PC12 of the
MPC low. This input is also paralleled with an interrupt input, IRQ6, for
rapid software response from the MPC.
As the E-lead can be sourced from a wide range of voltages, it is necessary
to have some form of current limiter in series with the opto-coupler, so that
its maximum current rating of 50mA is not exceeded. A bi-directional
current limiter circuit comprising transistors Q500, Q503 and Q504
accomplishes this, limiting the maximum current to approximately 6mA.
Back-to-back LEDs are used in DS500b so that the E-lead interface
responds to current flow in either direction.
Considering first the Ea lead being positive with respect to the Eb lead,
transistor Q503 is turned on by base resistors R512 and R513. Current
flows through one LED of DS500b, diode D500a, transistor Q503 and
resistor R514 before returning through the Eb lead. When the current flow
is such that the voltage drop across R514 reaches approximately 600mV,
NPN transistor Q504a starts conducting and diverts base current from
Q503. With less base current, Q503 progressively comes out of saturation,
limits the current flow, and acts as a current limit.
Operation with the Ea and Eb lead polarities reversed is identical, except
that current flows through Q500 and D500b with PNP transistor Q504b
acting to control the current flow.
The E-lead incorporates protection and EMC filtering as for the M-lead.