Keithley 708A Instruction Manual Download

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IEEE-488 Programming

5-40

5.10 Relay command combinations

There are four device-dependent commands that have an im-
mediate effect on relay states:

• E0N... — Point to relays, open specified crosspoints.

• E0C... — Point to relays, close specified crosspoints.

• P0 — Open all relays.

• Zn,0 — Copy setup “n” to relays.

Combinations of these commands in the same command
string cause only one relay switching operation when the X
character is received. The command hierarchy (E, P, Z, N, C)
determines the final data that is sent to the relays. This is
shown in the following examples.

Example 1

The command string “E0P0CA1X” sets the edit pointer to
the present relay setup, opens all relays, and closes cross-
point A1. A1 will be the only closed crosspoint.

Example 2

The string “E0Z5,0CA1X” sets the edit pointer to the relays,
copies stored setup #5 to the relays, and closes crosspoint
A1. A1 will be closed regardless of the state of A1 in setup
#5. The status of the relays will be a combination of setup #5
and a closed A1 crosspoint.

Example 3

The command string “E0Z5,0NA1X” points to the relays,
copies setup #5 to the relays, and opens crosspoint A1. If set-
up #5 had specified A1 to be closed, the command NA1 over-
rides that. (A1 will not close, then open, as there will be only
one relay switching operation.) The relays will reflect setup
#5 and an open A1 crosspoint.

5.11 Timing considerations

Timing considerations for IEEE-488 programming include:

• Data transfer rates between the controller and Model

708A (stand-alone or master) over the IEEE-488 bus.

• Command string parse time within the stand-alone or

master unit.

• Data transfer rates among the units in a master/slave

loop.

• Execution times of the tasks defined by device-

dependent commands.

The times needed for these actions are determined by the
length of the command string, the number of units in a
master/slave configuration, the types of commands, and the
speed of the controller.

Typically, a command string sent to the Model 708A will
transmit at a rate of four characters per millisecond.
(Assuming the transfer speed of the controller does not affect
the listening rate of the Model 708A.) For example, the
CA1,CA5X command string will take 2ms to transmit from
the controller to the Model 708A.

When the Model 708A is sending data to the controller (e.g.,
uploading setup data), the transmission rate will typically be
2.5 characters per millisecond.

The bus hold-off time for each command is the time from re-
ceipt of the “X” to “instrument configured.” It includes the
parsing time, data transfers within a master/slave loop, and
command execution time.

Table 5-14 summarizes the total times (transmission plus
hold-off) for device-dependent commands acting on a stand-
alone unit. Table 5-15 summarizes these times for a master
and one slave system. Bus hold-off times for individual com-
mand strings can be calculated by subtracting the transmis-
sion time (four characters per millisecond).

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