Keithley 7707, User Manual

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Figure 15
Short circuit protection for digital outputs
K1, K2, K3, and K4 protection circuits
Each digital I/O channel has a circuit to protect it from high voltage. Figure 11 shows the key components of the protection
circuit for channel 11. It includes a zener clamp to limit voltage to 40V, and a fly-back diode to provide a current discharge
path for an inductive load (i.e., relay coil). Without a current discharge path, fly-back voltage that occurs when the inductive
load is switched could damage the digital I/O.
Figure 11 shows how to connect an external circuit to the K1 protection circuit for channel 11. This circuit is shown connected
to output line Bit 0, but it is also connected to the other output lines (Bits 1 through 7) in a similar fashion to protect those
lines. As shown in the Equivalent Circuit, the flyback diode is placed across the relay coil to provide a discharge path for fly-
back voltage. There are similar protection circuits for the other three digital I/O channels; K2 protects channel 12, K3 protects
channel 13, and K4 protects channel 14.
External protection circuits
For localized protection, an external flyback diode can be connected directly across the relay coil to provide a current
discharge path for flyback voltage.
Figure 13 shows how to connect the diode across the coil.
Even when using K1, K2, K3, or K4 protection, additional devices, such as fuses and current limiting resistors, may be
required to adequately protect the switching module from damage caused by reactive loads.
Output bit patterns
A binary bit pattern is a series of ‘0’s’ and ‘1’s’ that identifies the state of each output line (bit) in the pattern. Setting a bit to a
logic ‘1’ effectively sets the output line high to approxi4.3V while setting it to a logic ‘0’ sets the output line low to
0V.
Operations to set output bit patterns are divided into two categories; basic operation and advanced operation:
Basic operation — Basic operation handles each output channel as a single 8-bit output pattern. An 8-bit pattern is known as a
byte. Output bytes can be set from either the front panel or set using remote programming.
NOTE For most applications, it is only necessary to set one 8-bit output pattern (byte). When working
with bytes, you do not need to concern yourself with advanced operation.
Advanced operation — Advanced operation provides additional flexibility by allowing two digital output ports to be treated
as a single 16-bit output pattern (word), or all four output ports to be treated as a 32-bit output pattern (double-word). Words
and a double-word can only be set using remote programming.
Ground
Digital
1A Max
–
25V
CAUTION To avoid damage to digital
circuitry, DO NOT apply >25
directly to a digital I/O line.
Thermal
Limit
Output
+