Technical data

74 34980A Service Guide
3 Calibration Procedures
Relay Plug-in Modules
There are two methods you can use to verify relays:
• Read the relay cycle count.
• Measure the relay contact resistance.
Relay Cycle Count
The instrument has a Relay Maintenance System to help you predict relay
end- of- life. The instrument counts the cycles on each relay in the
instrument and stores the total count in non- volatile memory on each
switch module. You can use this feature on any of the relay modules and
the internal DMM.
• In addition to the channel relays, you can also query the count on
backplane relays and bank relays. Note that you cannot control the
state of these relays from the front panel but you can query the count.
• You can also query the state of the six relays on the internal DMM.
These relays open or close when a function or range is changed on a
module.
• You can reset the count but the instrument must be unsecured (see
“Calibration Security" on page 47 to unsecure the instrument).
• The 34923A, 34924A, and 34933A modules can be configured for 2- wire
(differential) or 1- wire (single ended) measurements. Since two coils are
required to drive each channel relay in the 2- wire mode, the module
stores the cycle count for each coil and returns the greater of the two.
To determine the cycle count for each coil, reconfigure the module for
the 1- wire mode (a power cycle is required) and query the count.
• The FET switches on the 34925A FET Multiplexer module have an
infinite life when used under normal operating conditions. Therefore,
the cycle count is not recorded and this command always returns “0”
(will not generate an error). Although the count on the FET switches is
not recorded, you can read the actual cycle count on the mechanical
Analog Bus relays.
• On the RF Multiplexer modules (34941A, 34942A), the signal path to
the COM terminal consists of both a channel relay and a bank relay.
For each bank, the module stores the cycle count for each channel
relay, the bank relay, and returns the greater of the two. For example,
to determine the cycle count on Channel 11, the module recalls the
count on Channel 11, the count on the Bank 1 relay, and returns the
greater of the two. In addition, the cycle count on the two channels
within the same physical relay package, will always be equal. Therefore,
the cycle count for Channels 11 and 12 will always be equal.