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A magnetic flowmeter transmitter includes a differential amplifier that senses an additive combination of actual flowmeter electrode outputs and simulated electrode outputs. The differential amplifier provides an additive amplifier output. A simulator provides the simulated electrode outputs. The si

A magnetic flowmeter transmitter includes a differential amplifier that senses an additive combination of actual flowmeter electrode outputs and simulated electrode outputs. The differential amplifier provides an additive amplifier output. A simulator provides the simulated electrode outputs. The simulator senses the additive amplifier output and provides an error indication when the additive amplifier output is outside a normal range.

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What is claimed is: 1. A magnetic flowmeter transmitter, comprising: a differential amplifier sensing an additive combination of actual flowmeter electrode outputs and simulated electrode outputs, the differential amplifier providing an additive amplifier output; and a simulator that provides the s

What is claimed is: 1. A magnetic flowmeter transmitter, comprising: a differential amplifier sensing an additive combination of actual flowmeter electrode outputs and simulated electrode outputs, the differential amplifier providing an additive amplifier output; and a simulator that provides the simulated electrode outputs, the simulator sensing the additive amplifier output and providing an error indication when the additive amplifier output is outside a normal range. 2. The magnetic flowmeter transmitter of claim 1 wherein the differential amplifier comprises: a first amplifier having a first input coupling to a first flowtube electrode contact and having a second input coupling to a first simulated electrode output; and a second amplifier having a third input coupling to a second flowtube electrode contact and a fourth input coupling to a second simulated electrode output. 3. The magnetic flowmeter transmitter of claim 2 wherein the simulator comprises: resistors coupling the first and second simulated outputs to the second and fourth inputs. 4. The magnetic flowmeter transmitter of claim 3 further comprising: feedback networks coupled to the first and second amplifiers, the resistors and feedback networks forming resistive voltage dividers that reduce the simulated outputs at the second and fourth inputs. 5. The magnetic flowmeter transmitter of claim 2 wherein the flow sensing circuit tests active electronics in the magnetic flowmeter transmitter. 6. The magnetic flowmeter transmitter of claim 2 wherein the simulator further comprises AND gates that generate the simulated electrode outputs. 7. the magnetic flow transmitter of claim 2 wherein the simulator receives samples of the outputs of the first and second amplifiers for sensing the response. 8. The magnetic flowmeter transmitter of claim 1 wherein the simulator comprises: a flow sensing circuit that senses a response to the simulated outputs and provides the error indication. 9. The magnetic flowmeter transmitter of claim 8 wherein the flow sensing circuit compares the response to a normal range and provides the error indication when the response is outside the normal range. 10. The magnetic flowmeter transmitter of claim 1 wherein the simulator provides the simulated electrode outputs automatically. 11. The magnetic flowmeter transmitter of claim 1 wherein the simulator provides the simulated electrode outputs in response to a user request. 12. The magnetic flowmeter transmitter of claim 1 further comprising an analog-to-digital converter receiving the additive amplifier output and providing a stream of digital representations of the additive amplifier output to the simulator. 13. The magnetic flow transmitter of claim 1 wherein the flow transmitter comprises measurement cycles and the simulated electrode outputs are generated every M measurement cycles where M is an integer. 14. A method of testing a magnetic flowmeter transmitter, comprising: providing simulated electrode outputs; providing a differential amplifier sensing an additive combination of actual flowmeter electrode outputs and the simulated electrode outputs; providing an additive amplifier output from the differential amplifier; and sensing the additive amplifier output; and providing an error indication when the additive amplifier output is outside a normal range. 15. The method of claim 14 further comprising: coupling resistors from the simulated outputs to the differential amplifier. 16. The method of claim 15 further comprising: providing feedback networks in the differential amplifier, the resistors and feedback networks forming resistive voltage dividers to reduce the simulated outputs as sensed by the differential amplifier. 17. The method of claim 14 further comprising: sensing the additive amplifier output with an analog-to-digital converter and a flow sensing circuit. 18. The method of claim 17 further comprising: comparing a digitized representation of the additive amplifier output to a normal range and providing the error indication when the response is outside the normal range. 19. The method of claim 14 further comprising: providing the simulated electrode outputs automatically. 20. The method of claim 14 further comprising: providing the simulated electrode outputs in response to a user request. 21. The method of claim 14 further comprising: testing active electronics in the magnetic flowmeter transmitter with the simulated electrode outputs. 22. A magnetic flowmeter transmitter, comprising: a first amplifier having first and second inputs, the first input coupling to a first flowtube electrode contact; a second amplifier having third and fourth inputs, the third input coupling to a second flowtube electrode contact; and a simulator that provides simulated electrode outputs to the second and fourth inputs, the simulator sensing a response to the simulated electrode outputs and providing an error indication when the sensed response is outside a normal range. 23. The magnetic flowmeter transmitter of claim 22 wherein the simulator comprises: resistors coupling the simulated outputs to the second and fourth inputs.