I've dealt with Honeywell's ISA100 system for several years and have talked to guys who run Wireless HART networks.

Whether Honeywell's implementation of ISA100 features spread to other vendors (Yokogawa) in the same fashion, I'm not sure, so I call it Honeywell's implementation of ISA100.

There are similarites with both networks:
- battery powered field instruments with hazardous ratings.
- Adapters that talk the respective protocol for wired devices that want to get on the wireless network
- Gateway sits on a plant's wired Ethernet network
- field data provided to the plant via OPC server or Modbus
- browser based network supervision

But there are two major (architectural) differences between the networks:
- single vs multiple radio access points.
- single vs multiple protocols.

1) single vs multiple radio access points

Wireless HART uses a combined radio access point/gateway. It's one box. In Wireless HART the gateway that provides data to the plant is the same box as the radio access point. In Wireless HART, there is only one radio access point per network. The gateway/radio access point is the 'network'.

ISA100 uses a DC powered separate radio access point and a separate gateway box that talks to the plant network.
There is only one gateway per 'network' but the gateway supports multiple radio access points.

Why does this matter? Battery life.

In Wireless HART, the battery powered field transmitters are 'repeaters' used in a 'mesh' connection that connects a field transmitter (whose signal is not strong enough to reach the gateway) to the gateway. There might be multiple hops through multiple repeaters to get the gateway. But only battery powered field transmitters can function as a repeater.

In Honeywell's ISA 100 system, the DC powered Field Access point (FDAP) can service about 30 field transmitters. An FDAP costs about the same as a field transmitter. So when a good sized area needs radio coverage, ISA can use more than one DC powered radio access points so that the battery powered field transmitters do not act as repeaters - rather they go to sleep, wake up, take a reading, shoot out the reading then go back to sleep and preserve their battery power. The DC powered field access point can be awake 24-7, taking data from the field transmitters.

Originally and up to about 3 years ago, Honeywell's ISA100 field transmitters would not act as meshing repeaters. But as of a given firmware update (about 2-3 years ago?), they now do, so in Honeywell's ISA100 implementation field transmitters an now eat up batteries as quickly as Wireless HART when acting as a repeater.

Battery life
Battery life is a monitored variable. I see a realistic two - three years battery life for ISA100 for non-meshing/non-repeating field transmitters.

Battery cost
Honeywell's ISA100 field transmitters use two standard, off-the-shelf, Lithium D-cell batteries in each field transmitter. A battery certain brand name and model number is needed for compliance with hazardous area rating. These are $25 D-cell batteries.

The gripe I've heard from the guys running Rosemount Wireless HART is that the Rosemount battery packs are proprietary and cost a pretty penny. This is barstool talk at an ISA show, but it's backwards from what you heard. I'll vouch for my 2-3 years. Take it for what it's worth.

2) Single vs multiple protocols
There's a channel in ISA100 that allows for customizing (at a manufacturing/engineering level, not at field level) a data protocol that is not typical field transmitter functionality, where typical means the typical set of HART data functions. I know that the Honeywell ISA100 implementation uses this now for two non-conventional protocols
- legacy serial bitmark data from the Enraf tank gauging system on the 'custom' channel.
- vibration data for SKF vibration nodes.

What's interesting is that I've heard that ISA100 now uses that channel to handle Wireless HART field transmitters. The way I heard it, this channel is supposed to support HART/IP from the Gateway. The only drawback is that Emerson's AMS does not use HART/IP because Wireless HART preceded HART/IP, which would limit AMS functionality through an ISA100 port. But for field data collection purposes, wireless HART transmitters are supposed to work on the latest (firmware) ISA100 network.

3) One difference in commissioning
- wireless HART needs manual entry of the network security key
- ISA100 generates the security key at the gateway and sends it wirelessly to the field device