Remote ACCES Overview
Distributed Input/Output is commonly used to reduce installation cost, “home run” wiring, and potential noise interference. The first step is to take the I/O circuits out of the computer and put them into separate enclosures. The second step is to distribute those enclosures as close to the sensors or end devices as possible.
One approach to this problem was to use solid state I/O modules such as described elsewhere in this web site. That approach had the advantage of relatively low cost. Also, in event of a failure, only one channel was lost and the failed module could be replaced at low cost. The disadvantage here was that the I/O mounting card is bulky and had to be packaged suitably for the environment where it will be installed. Another approach to the problem is to use pre-configured, multichannel I/O units that may have limited drive capability and channel-to-channel isolation. Generally, these units are more costly and, in event of a failure, the entire unit may be have to be replaced. An advantage to this approach is that stand alone operation is sometimes possible. Our REMOTE ACCES pods strike a compromise between these two approaches.
REMOTE ACCES pods are intelligent sensor-to-computer interface units for use with digital and analog signal devices that are remote from the computer. The pods are packaged in rugged NEMA 4 enclosures and can withstand harsh industrial and marine environments. Applications include, but are not limited to:
- Remote Data Acquisition
- Supervisory Control
- Building Automation
- Direct Digital Control
- Process Monitoring and Control
- Energy Management
- Security Systems
- Relay Control
Communication with the host computer is via two-wire, EIA RS-485, half-duplex asynchronous serial communications using ASCII-based command/ response protocol. Thus REMOTE ACCES pods can communicate with virtually any computer that has an RS-485 port. The two-wire connection keeps the number of cables and connectors to a minimum and simplifies installation and overall cost. Optical isolation at each pod prevents ground loop and surge problems. REMOTE ACCES pods can be used at cable distances through 4,000 feet. Each pod can be used as an autonomous unit or as many as 32 pods may be connected on a single two-wire, multi-drop RS-485 network. Each pod is assigned a unique address and communication uses a master/slave prot ocol. Each pod talks only if questioned by the computer.
Use of ASCII- based protocol lets you write applications in any high-level language that supports ASCII string functions: BASIC, QuickBASIC, C, C++, Pascal, Turbo-Pascal, etc.
A type 8031 microcontroller with 8Kx8 RAM, 8Kx8 non-volatile EEPROM, a crystal-controlled oscillator, and a watchdog timer circuit gives REMOTE ACCES pods the capability and versatility expected from a modern distributed control system. To accommodate special programs, the RAM and EEPROM memory can each be expanded to 32Kx8. A watchdog timer resets the pod if, for some unexpected reason, the microcontroller “hangs up” or if the power supply voltage drops below 4.75 VDC.
Data collected by the pod (or output control signals) are stored in local RAM and can be accessed later by the host computer. This facilitates stand-alone operation.
The pod’s network address is programmable from 00 to FF (hex) and the assigned address is stored in EEPROM and saved as the default address at the next power-on. Similarly, the baud rate is programmable for 1200, 2400, 4800, 9600, 14.4K, 19.2K, and 28.8K. (Note: If the communication cable is short, then 56K baud can be realized if the opto-isolators are bypassed.) The programmed baud rate is also stored in EEPROM and used as the default value at the next power-on.
The command structure is seven bits, even parity, one stop bit. All numbers sent to or received from the pod are in hexadecimal form. There is a lot of commonality in the command lists for REMOTE ACCES pods but each pod also has commands unique to its particular function. (ACCES provides detailed documentation and software with the pods.) Commands are in the form of simple letter/hex-number combinations and commands are terminated with a CR (carriage return). For example, the address command for a pod in a network is:
xx is the pod address from 0l to FF (hex) and [CR] is ASCII character #l3.
A nice programming feature of the REMOTE ACCES Series pods is that an address command need be sent only once during communication between the pod and the host computer. It enables communication with that specific pod and disables communication with all other pods on the network until a new address command is issued. Other command examples (these examples are taken from the command lists of models RIOD24 and RDG-24) are:
Output a logic high on bit x.
Set digital input active state low on bit xx.
Set digital output xx to free run with period yy.
All commands have an acknowledgment response.
The pods contain CMOS low-power circuitry, an optically-isolated receiver/transmitter and voltage regulators for local and external isolated power. Thus two power sources are required: isolated power and local power. The isolated section of the pod accepts 7.5 to 35 VDC and uses only a little power, about seven milliamperes. The voltage could be provided by the host computer’s +12 VDC supply. Voltage drop in the cable is insignificant because current demand is so low.
Power for the remainder of the pod’s circuitry can be provided by a separate local power supply. Only 7.5 VDC is needed but, because on-board voltage regulators are provided, the local power supply voltage can be as high as 16 VDC. Current required is a maximum of 150 milliamperes. Most REMOTE ACCES pod types require less than 100 milliamperes. Some pods, notably digital I/O pods, can interface to application-specific voltages as high as 50 VDC. In the case of digital outputs, the application voltage must also be supplied for use by the pod.
There is a screw terminal assembly inside the top cover of the pods and a gland is provided to seal wiring for full NEMA compliance. You can assemble a cable of whatever length you need, route it through the gland, and terminate the cable at the other end by whatever termination method best fits your application. If you wish, ACCES can provide a custom cable fabricated according to your needs. Give us a call.
The enclosure is a sealed, die-cast aluminum-alloy that can be easily mounted using two long M-3.5 x 0.236 screws that are provided. Multiple enclosures can be mounted upon each other in a stack. The cover incorporates a recessed neoprene gasket and is secured to the body by four recessed M-4 stainless steel captive screws. Mounting holes and cover-attaching screws are outside the sealed area to prevent ingress of moisture and dirt. Four threaded bosses inside the enclosure provide for mounting the printed circuit card assemblies.