Electrical Parameters

The electrical parameters of this serial datalink are a modification of the EIA RS-485 standard. In some areas, this RP conflicts with RS-485. This RP shall serve as the guiding document in such cases. APPENDIX A  details a serial databus standard node which defines the interface circuit parameters. Operation of this standard node is detailed in this section.

Logic State -  Positive true logic will be used when referring to the states of transmitted inputs and received outputs. Referring to APPENDIX A,  the input of the transmitter (marked as point Tx) and the output of the receiver (marked as Rx) will be in logic state 1 when driven or passively pulled to +V, and will be at logic 0 state when driven to ground.

Bus State -  The bus is in a logic 1 (high) state whenever Point A is at least 0.2 V more positive than point B. See Figure The bus is in a logic 0 (low) state whenever Point A is least 0.2 V more negative than Point B. The bus state is indeterminate when the differential voltage is less than 0.2V.

Logic High State -  The bus will be in a logic 1 (high) state when all connected transmitters are idle or sending logic 1. An idle state is produced when all transmitters in the circuit are turned off. All nodes shall include a means to pull the bus to logic 1 (high) when all transmitters are off (see APPENDIX A ).

Logic Low State -  The bus will be in a logic 0 (low) state when one or more transmitters are sending logic 0, which guarantees that logic 0 (low) dominates when the bus is in contention.

Network Capacity -  The bus will support a minimum of 20 standard nodes where each node is comprised of the circuit defined in APPENDIX A.  Deviations from this circuit must be carefully analyzed to determine the impact on bus loading and noise margins over the common mode range.

Bus Termination -  Bus termination resistors referenced in RS-485 are not required and shall not be used.

Ground -  All assemblies using the link must have a common ground reference.

Wire -  Aminimum of 18 gauge twisted pair wire, with a minimum of one 360° twist per inch (2.54 cm), is required. The twists shall be distributed evenly over the length of the wire.

Length -  This RP is intended for, but not limited to, applications with a maximum length of 130 ft. (40 m.)