OutGuard (OutG) InGuard (InG) multimeter hardware and firmware definitions

[Kiriakos GR]

You are here to find out more about multimeter terminology of OutGuard (OutG) and Inguard (InG) which this is relative to benchtop multimeter product design.

I bet that your search started because of your discovery that such words (descriptions) they are mentioned in the firmware status screen of your benchtop multimeter, or because a relative message pop up as Error message in the screen of your benchtop multimeter.

To start with regarding definitions I must say that Out-Guard & In-Guard they are circuits, these circuits communicate and exchange data.
100% similar concept in hardware circuit this is USB or Serial to infrared LED adapter for handheld multimeter data logging.

Now with a single USB or Serial to infrared LED adapter, one multimeter will communicate with windows PC by transferring a small quantity of data per second.
Such data (data logging session) will be exchanged once every second or in a slower interval.
In the end of the day, such communication involving a pair of infrared LED this is a Guard maintaining communication and delivering at the same time electrical isolation between point A and point B.

In the world of benchtop multimeter high test system throughput is vital to maintaining production capacity and therefore data exchange performance due such infrared LED circuit this must be increased.

Originally HP multimeter manufacturer implemented such (OutGuard & Inguard) circuits in their HP3458A at 1989.

Inguard circuit this is about Isolation of the ADC hardware be isolated from the controlling microprocessor (transferring readings, and parsing commands, the microprocessor has been busy scanning the front-panel keyboard, updating the display, and polling various peripheral ICs to monitor and update status information.)

OutGuard circuit this is about Isolation mainly of IEEE 488 GPIB (General Purpose Interface Bus), such implementation offer electrical isolation between PC systems and the multimeter it self and the benefit this is floating voltage measurements’ which a grounded to earth multimeter this cannot perform.

In the world of handheld multimeter we think floating voltage measurements’ as given advantage, but benchtop multimeter cannot act the same with out additional circuitries and Guards or Protectors of their valuable electronic components.


Now in the question of why a (OutGuard & Inguard) in hardware implementation this requiring other than electronic components and software support in firmware?
The answer which I am offering this is that Point to Point data transfer’s throughput this is always relative to what called as data compression algorithm.
Infrared LED alone this can handle Serial Port bandwidth which this is not that much, but with help of data compression and smart algorithms data transfer increases.

In the end of the day everything is related to production cost, for example if such (OutGuard & Inguard) circuits requiring optical fiber isolation they can be build with technology of 2019 in which neither bandwidth and neither speed this is nowadays an issue.
From the other hand use of high-end data transfer in to a multimeter this will increase production cost and retail cost.


HP3458A 8 ½ multimeter (the gold standard since 1989) this is still alive as Agilent / Keysight 3458A.
OutGuard & Inguard circuits they do exist as technology even at Fluke 8842A multimeter, OutGuard &  Inguard as circuit design this has be further improved, Fluke 8856A and DMM4050 these using latest know revision of this technology.

[Andrew-88]

I am no electronics designer, but seems unwise such circuits coexisting in a single PCB.

If they were at separate PCB's then this would be a great help in favor of repair-ability.
By simply replacing the faulty board.