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Power Wiring

Communicating with BASIC I/O

  • Multidrop
  • Repeat
  • RS-422
  • RS-485

RS-485 programming

BASIC I/O Hardware User Manual

Chapter 2a - Mounting

BIO4, BIO8 and BIO16 (not AD) Products

BASIC I/Os come in 4, 8, and 16 channel versions. Figure 2-1 below shows the footprint of each BASIC I/O. Using the corner holes, the unit can be mounted with 4 - #6 or #8 round head or pan head screws. The BIO16 version has two additional mounting holes located near the center of the board as well. Hole locations in relation to the overall dimensions for each are shown below. Since the same BASIC I/O boards are used for digital or analog applications, the same mounting dimensions and panel space are used for both.

BIO4, BIO8 and BIO16 Footprints

Figure 2-1 BASIC I/O Footprint

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Power Wiring:

Power connections are made at the 2 position terminal block located on the module board marked +5V and GND No. 8 captive wire clamps accept 10-16 AWG wire or spade lugs.

Figure 2-2 BASIC I/O DC Power Wiring

Figure 2-2 DC Power Wiring

Power Wiring Conventions
+ of the power source to the terminal marked +5V
-  of the power source to the terminal marked GND

Power Requirements
Voltage: 5.0 - 5.4V
Current: 250mA + 25mA Per digital module or 250mA + 200mA per Analog module.
Note that the current draw of some specialized digital modules, such as the IDC5S or The IDC5Z, can be
substantially larger than 25mA. Consult the specific module data sheet for this value when sizing power supplies.
Practices: In general it is good practice to reserve the +5V power supply exclusively for the task of powering one or more units. As with any microprocessor based equipment, reasonably clean power is required for reliable operation.
Sharing power with other devices such as field signal transducers and contact excitation should be strictly avoided.

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Communicating with BASIC I/O

The BASIC I/O is designed to serve as an intelligent I/O front-end for a Host computer (typically a PC). The Host and communicate over a serial link. This interchange is half-duplex in nature; that is to say the Host and BASIC I/O will never be transmitting at exactly the same time. Further, the communications protocol is considered "speak-only-when-spoken-to"; the Host must poll the BASIC I/O whenever it needs fresh data. This polling is accomplished when the Host sends an instruction to the BASIC I/O. The BASIC I/O will then generate a reply. Each valid instruction will elicit a corresponding response. The integrity of this communication is verified using message content checksums.

The serial communication is in the form of ASCII printable characters and makes heavy use of the hexadecimal numbering system. The format of the ASCII characters used is: one start bit, eight data bits, one stop bit, and no parity.

BASIC I/Os can be networked together to obtain up to 4096 I/O points of data. These serial networks can be either multidrop or repeat.

Figure 2-3 Multidrop vs. Repeat Networks

Figure 2-3 Multidrop vs. Repeat Networks

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Multidrop: Multidrop networks can be up to 5000ft long end-to-end.

Each station is passively located on the network and represents one "Drop" or load to the host communication driver.

A multidrop network will tolerate the loss of power to any one station without effecting the rest of the network.

RS-485 can only be multidrop.

Signal boost may be necessary depending upon line conditions and number of drops.

Repeat: Repeat networks can be as long as 5000ft between each unit.

Each station plays an active roll in communications to other units. If power is removed from a unit in a Repeat network, communications to units "downstream" from the host will be lost as well.

The serial communication link between a Host computer and a network of BASIC I/Os is made up of either a single (RS-485 half duplex) or dual (RS-422 full duplex) shielded twisted pair(s) of wires whose shield(s) are connected to a signal common conductor. This communication link should in turn have an overall shield which is isolated from the signals (including signal ground) and connected to Earth or Chassis ground at one location. The most common cause of difficulty experienced by customers in the field is improperly installed communication wiring.

RS-422 Advantages
  • Easier to implement in software since host driver need not be controlled.
  • Can be either multidrop or repeat. 
  • No turn-around delay required.
  • Requires five conductor wire instead of three.
RS-485 Advantages
  • Needs only three conductor wire.
  • Host 485 driver control must be implemented requiring tricky serial port manipulations.
  • Can only be multidrop. 
  • Usually requires turn-around delay implementation.

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RS-485 Programming

The BASIC I/O will work equally well when connected to either RS-422 or RS-485; however, special Host programming considerations may be necessary when implementing an RS-485 network.

Unlike RS-422 where both the Transmit Receive signals have their own differential pair of conductors, RS-485 utilizes only one differential pair. The single pair of conductors is used bidirectionally and handles both the Transmit and Receive signals. In order for this to be possible, the transmitter for each device on this type of network must be enabled and disabled whenever a transaction between devices is to take place. The BASIC I/O has been designed to handle this control automatically; however, the transmitter control for most popular RS-485 cards that are installed in the Host computer must be controlled by the user program. This control is not straight forward and may impact the overall system throughput with inherent delay periods.

The following is a typical instruction/response transaction between a Host computer and a BASIC I/O using RS-485:

  1. The Host computer enables it's RS-485 transmitter (usually via the RTS signal line).
  2. The Host then sends an instruction to the BASIC I/O in the form of an ASCII printable string.
  3. Once the Host determines that the string has been completely sent, the RS-485 transmitter is disabled.
  4. Every BASIC I/O on the network receives the instruction and begin to decode it. The particular BASIC I/O addressed begins to construct a response.
  5. Once the Carriage return of the instruction is received, the BASIC I/O begins to transmit the response.
  6. The Host receives the response and takes the appropriate action.

This interaction is heavily dependent upon asynchronous timing. Usually, the Host software has no real means of determining that the instruction has been completely sent. This means that the program must calculate the approximate time necessary to transmit the entire instruction before the RS-485 driver is disabled. Since the BASIC I/O can respond very quickly to the instruction, the host must disable the driver as soon as possible in order to receive the BASIC I/Os' response. To cope with this dilemma, the host can configure the BASIC I/O to delay a short period  of time before transmitting it's response.

RS-485 communication can be tricky at best, and should be seriously considered before being adopted. Third party software users should be certain the package they have chosen supports the particular RS-485 communication card to be used.

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