Accessors are the communication links between all field devices and the Central Processing Unit. This communication is made in response to one of several commands, but is entirely automatic. Each Accessor can be uniquely addressed, eliminating the possibility of information mix-up. In addition to analog values or binary status information, the Accessor also transmits information identifying itself to the Central Processing Unit to facilitate information processing and to protect against errors introduced on the Accessor trunk due to high electro-magnetic interference.

The Type B34 Accessor is used to monitor the status condition of field-mounted contacts. Each B34 Accessor can monitor the status of 4 contacts. These may be any combination of Normally Open (NO) or Normally Closed (NC) dry contact switches. For monitoring powered contacts, an isolator board option is available. The Type B34 Accessor can detect "Off Normal" contact status, when the change of state duration is 1 m/sec for dry contacts and 20 m/sec for A.C. powered contacts. The Type B34 Accessor may be placed anywhere along the Accessor trunk and occupies one address and one position in any applicable Accessor housing. The contact sensor may be located up to 100 feet (30 m) from the Accessor.