Profibus PA and Foundation Fieldbus — a cost comparison

There have been many comparisons between Foundation Fieldbus (FF) and Profibus for use on field instruments in the process industries. If you put the two protocols side by side you will see that either will do the job and provide the much published cost savings.

Both systems provide you with:

• bi-directional communications;
• information on the process variable;
• alerts;
• methods to communicate with the instrument over the bus; and
• bus power and intrinsically safe (IS) capabilities.

They use the same wiring and all major distributed control system (DCS) vendors now support both Profibus and FF, so it is the user’s decision to choose the best fit.

In all the comparisons between Profibus and FF that I have read, none discuss the cost differences between the two solutions. A comparison is particularly difficult because the design and size of the project have a large impact on the outcome. So we need to look at some of the fundamental differences between the two protocols, and how they affect costs.

What is the main difference? Both Profibus PA (PA) and Foundation Fieldbus started off in the same working group and have much in common. The main difference is where the line between the field instrument and the control system is drawn (see Figure 1). In PA, the input and output blocks located in the DCS in a conventional system are moved into the field instruments. However, no ‘control’ blocks are moved into the instruments. All control is done in the DCS. In FF, the input and output blocks, along with control blocks, have been moved into the field instruments. Consequently ‘control in the field’ is the fundamental difference.

Figure 1: The line between control system and instrument for Profibus PA and Foundation Fieldbus.

Stack size, power budget and unit costs

Siemens has been in the process of adding FF to products that already have PA capabilities. When Siemens started the project, the company anticipated that since FF and PA both use the same physical layer and ASIC, the PA stack had to be removed and replaced with the FF stack. In addition, a small amount of memory had to be added because the FF is larger than the PA stack. As it turns out, the FF stack is about 350 Kb, while the PA stack is about 50 Kb (See Figure 2). Also, in order to execute all this added code, the clock speed would have to be doubled.

This means that the manufacturing cost for each FF device will be higher than the PA device. When comparing the list prices of several vendors of level products, the FF devices are about US$100 more than the same PA devices. The increased memory and clock speed also have a second effect: the power requirement on the bus has to increase. We noticed about a 2 mA increase in the power requirement for the FF device. For an end user, this is not an issue unless it affects the number of devices that can be put on a segment, thereby adding to the capital cost.

Figure 2: The protocols stack for Profibus PA and Foundation Fieldbus.

Number of devices per loop

The number of the devices per loop varies according to the overall design requirements of the project and whether it is intrinsically safe (IS). If the loop is non-IS, then the FF System Engineering Guidelines, recommend no more than 12 devices per loop. The macrocycle for this type of system depends on the function blocks being used, but it is generally between 0.5 and one second long. For the PA system, the general recommendation is not to exceed 24 devices per loop. The device update time for 24 devices is around 400 ms. Being able to put 24 devices on a loop means that you can put double the amount of instruments per loop in PA than you can in FF. This reduces the number of linking devices required for a PA site by half.

Some control loops have a requirement that the control loop update times are under 300 ms. Foundation Fieldbus recommends having no more than three devices per loop and use control in the field. For Profibus, it depends on the type of instruments used and the data requirements, but typically you could have 15 devices per loop and it would still meet a total control loop update time of 300 ms. This cuts the number of linking devices required for Profibus by five times.

Cost of linking devices

In both PA and FF, there are a number of vendors with different options for linking up to segments. In all cases, though, the cost of implementing an FF device is higher due to the complexity of the protocol, eg, an HSE to H1 linking device cost US$500 more than a Profibus DP to PA linking device sold by a major vendor.

Wiring and junction boxes

The wires and junction boxes are the same for PA and FF, except that you need more runs for FF. The increased number of loops means that you need more wires and more junction boxes, which increases overall costs.

Managing complexity

In Figure 2, we showed how much larger the FF stack is compared to the PA stack. The increase in the protocol stack is mainly due to the overhead of the control block for control in the field functionality. This adds complexity that has to be managed throughout the project. End users and company technical support groups reported that PA is easy to use compared to FF.

Another point to consider is that if you choose PA for field instruments, and use Profibus DP for drives and I/O, then your staff will be dealing with one protocol (one data packet) and one set of troubleshooting equipment. If you choose FF, then your staff will have to learn two protocols, because FF cannot support drives. This will increase overall costs. Interestingly enough, the protocol of choice for a secondary bus for FF installations is Profibus DP.

Example: Tank farm with 100 tanks

Foundation Fieldbus solution

	Maximum tanks per loop	= 12
	Loops	= 100/12 = 9
	Linking device costsa:	$3300 each
	Device added priceb:	$100 each

Profibus solution

	Maximum tanks per loop	= 24
	Loops	= 100/24 = 5
	Linking device costs:	$2750 each
	Device added price:	$0

Cost difference

	Cost difference	= cost(FF) – cost(PA)
		= (9*$3300 + 100*$100) – (5*$2750 + 100*$0)
		= $39,700 – $13,750
		= $25,950

From a purely capital cost point of view, implementation of the PA solution is about $26,000 cheaper.

Conclusions

From our analysis we have shown how:

• Profibus devices are less expensive;
• you can fit far more PA devices on one link;
• PA links are less expensive; and
• PA is less complex.

If you need control in the field, then FF is your protocol. Otherwise, PA is the far more economical solution.

 

*James Powell, P Eng, is product manager, communications systems, with Siemens Milltronics Process Instruments Inc.

Siemens Ltd
www.siemens.com.au

Footnotes
a. This is the cost of device(s) that will connect the Fieldbus to the high-speed bus. In FF, it would be a linking devices plus a power supply and power conditioner. In Profibus, it would be the cost of a DP/PA coupler and power supply. There are several different solutions for how to connect Fieldbus segments to a high-speed network. For this example, we have picked a major manufacturer who offers both a PA and FF Linking device in order to get a good comparison.
b. ‘Device added price’ refers to the additional cost that a manufacturer will charge for a FF device over a PA device. As stated earlier, FF level devices typically are about $100 more than the same PA level device.

References
FuRIOS 2 study — Fieldbus Ready for Practical Use
Foundation Fieldbus System Engineering guidelines AG-181