Agilent 7820 GC Service
173
You can synchronize a maximum of ten instruments using
Remote cables. Control of analysis is maintained by instrument
readiness - READY for next analysis, followed by START of run
and optional STOP of run triggered on the respective lines. In
addition, PREPARE may be issued to initiate pre-run activities.
All devices connected to APG remote are connected in parallel.
For example, if one device is NOT READY then all devices on
the bus are also NOT READY.
Electrical specifications
The APG signals are a modified open collector type. The signal
levels are generally TTL levels (low voltage is logic zero, high
voltage is logic one) but the open circuit voltage will be between
2.5 to 3.7 Volts. The typical voltage is 3 Volts. A voltage over
2.2 volts will be interpreted as a high logic state while a voltage
below 0.4 volts will be interpreted as a low logic state. These
levels provide some margin over the specifications of the devices
used.
The pull-up resistance, connected to the open-circuit voltage, is
in the range of about 1K ohms to 1.5K ohms. For a logic-low
state, for a single device on the bus, the minimum current you
must be able to sink is 3.3 milliamps. Since devices are
connected in parallel, when you have multiple devices this
minimum current must be multiplied by the number of devices
attached on the bus. The maximum voltage for a low-input
state = 0.4V.
The bus is passively pulled high. Leakage current out of a port
must be less than 0.2 milliamps to keep the voltage from being
pulled lower than 2.2 volts. Higher leakage current may cause
the state to be interpreted as a low.
Over-voltage protection - APG Remote connections are clamped
by a zener diode to 5.6 Volts. Exceeding this voltage will damage
the circuit (main board).
Suggested Drive Circuits
A signal on the APG bus may be driven by another APG device
or by one of the following circuits.
A relay, with one side connected to ground, when closed will set
a logic-low state.
An NPN transistor, with the emitter connected to ground and the
collector connected to the signal line will set a logic-low state if
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