Keithley 708A Instruction Manual Download

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Operation

4-11

4.7.1 Programmed settling times

The programmed settling time is a variable switching delay
that can be used to lengthen the fixed delay of the relay
(hardware) settling time. You can select, in 1ms increments,
up to 65 seconds of additional switching delay.

If an additional trigger is received during this time, it is pro-
cessed and the ERR LED is lit. At the end of the programmed
settling time, the Model 708A sets the MATRIX READY
output true.

To view the programmed settling time, use the U0 — status
command (paragraph 5.9.20).

NOTE

The U6 command sends the longest hard-
ware settling time for all matrix cards, and
the U0 command sends the machine status
word. The machine status word contains
programmed settling times.

To change the programmed settling time, use the S — pro-
grammed settling time (paragraph 5.9.18). Use a value be-
tween 0-65000.

The programmed settling time is in effect for all crosspoint
relay open or close operations until it is reprogrammed. Its
effect on trigger response times is described in paragraph
4.8.

4.7.2 Make/break and break/make rows

Make-before-break switching of relays is defined as con-
necting a new circuit before disconnecting the present cir-
cuit. It is used to eliminate transients caused by switching
between current sources. Break-before-make switching
means to disconnect the present circuit before connecting a
new circuit. It is used to avoid momentary shorting of two
voltage sources. Both of these switching operations are sup-
ported by the Model 708A.

Rows of crosspoint relays are user-selectable for make/
break, break/make, or don’t care operation. The selections
will be in effect for all switching until new choices are made.
When make/break or break/make operation is chosen, the
Model 708A automatically switches the crosspoint relays
through intermediate setups to perform the following steps:

1. Crosspoints in break/make rows are opened.
2. Crosspoints in make/break rows are closed.
3. Crosspoints in make/break rows are opened.
4. Crosspoints in break/make rows are closed; crosspoints

in don’t care rows are opened or closed accordingly.

These steps are apparent to the user except for the increased
settling time. If either make/break or break/make rows are
not selected, the appropriate steps in the previous list are de-
leted and the total settling time decreases. Because make/
break and break/make operations affect settling times and
trigger response, these operations are further discussed in
paragraph 4.8.

To change the status to break/make, make/break, or don’t
care, refer to paragraphs 5.9.21 and 5.9.22. Selecting a row
for make/break de-selects it for break/make and vice versa.
The row selection is in effect for all units connected in a mas-
ter/slave configuration. The operations are listed in Table 4-5.

When switching current sources, use make/break operation
to keep current flowing and eliminate switching transients.
When switching voltage sources, use break/make operation
to avoid momentary shorting of two paths together.

4.8

Triggering

When a Model 708A stand-alone or master unit is triggered,
the stored relay setup from RELAY STEP+1 is sent to the re-
lays. Triggers are enabled using the F1 command (paragraph
5.9.6).

The maximum trigger rate is specified with no make/break or
break/make rows selected. As described in paragraph 4.8.2,
additional switching delays are necessary with make/break
or break/make operation.

Table 4-5

Make/break and break/make operation

Present state

Action

Next state

Don’t Care

Make/Break

Break/Make

Select Make/Break
Select Break/Make
Select Break/Make
De-select Make/Break
Select Make/Break
De-select Break/Make

Make/Break
Break/Make
Break/Make
Don’t Care
Make/Break
Don’t Care

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