The MINIMUX® II is a microprocessor-based eight channel analog input scanner/multipexer and annunciator that provides low-cost automatic switching for multi-point display and alarm systems. Each MINIMUX® II can automatically switch up to eight inputs to another device such as a digital panel meter, controller, or PLC. Signal switching is done via reed relays making the MINIMUX® II ideal for switching thermocouples, RTDs, control loops, and AC & DC signals
In its most basic form, the PD138 is the electrical equivalent of a double-pole, eight position automatic switch. Whatever input is brought into the PD138 is dwelled on for a certain amount of time while being channeled/switched out to another device. For example, a typical application may involve channeling/switching eight type J thermocouples into a single digital panel meter.
Temperature Scanning
One of the most popular uses for the PD138 is temperature monitoring, especially the monitoring of bearing and stator winding temperatures. Any of the many different style thermocouples, RTDs, or thermistors can be used with the PD138 - right out of the box.
Automatic or Manual Scan
The PD138 automatically scans through each active channel, dwells on that channel for an independently programmed dwell time, while at the same time switching the signal to an external device. The scanning can be stopped, or scanned manually via the front panel stop/go pushbutton.
Alarm Annunciator
One of the key features of the PD138 is its ability to indicate and process alarms that are generated by an external device. Each channel of the MINIMUX® II has an independent ‘Alarm-in’ terminal that allows an external device to trigger an alarm on the MINIMUX® II. The external device may be the relays on a digital panel meter, such as the PD765, or the digital output from a PLC.
First-Out (Sequence F2A)
The PD138 can be configured to recognize first-out alarms. On the PD138, these are one or more alarms that occur during the first complete scan where an alarm is present on one or more channels.
When the PD138 is powered, the LEDs are turned on one channel at a time until all green and then all red LEDs have been tested. The internal horn will sound at the end of the LED test.
In the following application example, four oven thermocouples are scanned and channeled to a PD765 Temperature Meter where the PV is displayed and checked for low or high alarm conditions. All four ovens share the same low and high alarms, so the alarm inputs are joined together on the PD138
The PD138 MINIMUX® II is programmed in the following manner to satisfy this application:
Select alarm Sequence A
Stop-on-alarm is off
Program dwell times:
Channel 1 for ten seconds
Channel 2 for five seconds
Channel 3 for fifteen seconds
Channel 4 for five seconds
Channels 5-8 are disabled
The PD765-6R2-00 Trident is programmed in the following manner to satisfy this level application:
Select T/C
Select °F
Program alarm set and reset points:
The MINIMUX® II scans each input and reflects the alarm state. The PD138 shown above is scanning input 1 (Green LED) allowing the Trident to display the current temperature of 250°. No alarm is present since the temperature is within limits. The MINIMUX® II only responds to new alarm conditions detected in the currently active channel. The PD138 also avoids false alarms by detecting alarm conditions only after dwelling on a channel for 2 seconds, or 0.05 seconds before it has been programmed to go onto the next channel if the dwell time had been programmed for less than 2 seconds. This gives the PD765 time to stabilize and accurately display the temperature and to check for alarm conditions
The MINIMUX® II switches to (scans) input 2 channeling it to the PD765-6R2-00 where a temperature of 345° is found to cause a high alarm condition. The PD138 is programmed for Sequence A, so the channel 2 LED alternately flashes yellow and green, the internal horn sounds, and the PD138’s alarm relay transfers causing an external horn to sound. After acknowledging the alarm the channel LED will glow a steady yellow (while selected), or steady red (when not selected) until reset
The MINIMUX® II switches to (scans) input 3 channeling it to the PD765-6R2-00 where a temperature of 220° is found to cause a low alarm condition 2 seconds into the dwell time. Channel 3 LED alternately flashes yellow and green (shown yellow), the internal horn sounds and the PD138’s alarm relay transfers causing an external horn to sound.
Three-Wire RTD Inputs
Three-wire inputs such as thermistors and RTDs can be handled by switching the two common wires and connecting the third wire of each sensor to the RTD-TP terminal and to the temperature meter
Connect like color leads to terminals 1A & 1B, 2A & 2B, 3A & 3B, etc
Connect odd color leads to terminal TP
Connect outputs A, B and TP to the temperature meter
The example below shows three 3-wire RTDs multiplexed into 1 temperature meter
Thermocouple Inputs
Thermocouple wire must be used between the MINIMUX® II and the meter. This example shows 3 type J thermocouples multiplexed into 1 temperature meter
4-20 mA Inputs
A special version of the MINIMUX® II is used to switch 4-20 mA signals. The PD138-X-CL (X=power), has a built-in resistor network that eliminates loop interruptions. The process meter must be setup for 1-5 V input and the complete system should be calibrated together
The PD6000’s internal transmitter power supply may be used to power the transmitters. However, only the transmitter that is currently selected will be powered and all others will not be powered. Care should be taken to provide the transmitter with sufficient time to stabilize to provide an accurate output
Multiple MINIMUX® II can be connected together to increase multiplexing capacity for an unlimited number of points. When connecting several units together, one unit of the group is designated the master unit and all other units function as follower units. Multiple unit configuration is simply done by connecting the units together as shown below. The system will self-determine if there are two or more MINIMUX® II connected together
Each channel on the PD138 has its own Alarm-In line that provides independent alarm indication capability for each channel. The alarm input lines of the PD138 are normally high (pull-up to 5V). Relay contacts from a Precision Digital meter or a low level logic output from a PLC can be used to drive the PD138 alarm input lines
We packaged our most popular PD138 product combination for you at a significant savings vs. buying these components separately. The PDS178 is a kit that includes a PD765-6R2-00 Trident and PD138 MINIMUX® II in a PDA2821 NEMA 4X enclosure. The PDS178 is a Temperature or Process Scanning and Alarm System ready to wire into your application. You can’t beat the discounted package price!
Specifications Except where noted all specifications apply to operation at +25°C
General Number Of Channels: 8 double-pole channels per unit
Number Of Channels Per System: Unlimited
Signals Switching: DPST reed relays
Contact Resistance: 0.2 Ù maximum
Maximum Input Votage: 200 V (switched or common mode)
Maximum Current Switched: 0.5 A
Maximum Power Switched: 10 W
Dwell Time: Each channel adjustable from 0.6 to 30 seconds
Non-Volatile Memory: All programming values are stored in non-volatile memory for ten years if power is lost
Scan Method: Internal or manually controlled
Channel Indication: Green LED on front panel
Disabling Channels: Any channel may be disabled during setup
Scan Stop: The scan may be stopped by pressing and holding the STOP/GO button for more than 0.5 seconds. The scan may be resumed by pressing and releasing the STOP/GO button quickly (less than 0.5 seconds)
Alarm Input: Independent alarm input for each channel. Input Impedance; 25 Kohm, typical pull-up to 5 V
Alarm Sequence: Sequence A or Sequence F2A (first out)
Alarm Outputs: Alarm condition indicated by:
1. Front panel red LED for each channel
2. Relay, 1 SPDT (form C); rated 2 Amp @ 30 VDC or 2 Amp 2. @ 250 VAC resistive load; 1/14 HP @ 125 / 250 VAC for inductive loads. For failsafe operation, the relay is energized in the non-alarm state. In the case of a power failure, the relay will go to the alarm state, (NC contact is connected to common)
3. Built-in Horn, 85 dB
4. Stop-on-alarmed-channel (user select)
Alarm Acknowledge: Front panel ACK and rear connector terminals
External Switch Functions: The functions of the front panel buttons are available at screw terminals at the rear of the instrument
Message Labels: Free, custom printed, 2 lines per message at 14 characters per line. Factory or field printable. See manual for details
Connections: Removable screw terminal connectors provided:
Power + Relay: 22 to 12 AWG; Alarm Input, Analog Signal, External Switches, I/O: 30 to 16 AWG
Operating Temperature: -40°C to 65°C
Storage Temperature: -40°C to 85°C
Relative Humidity: 0 to 95% non-condensing
Power Options: 115 VAC, ± 10%, 50/60 Hz, 4 VA; 230 VAC, ± 10%, 50/60 Hz, 4 VA; 12-24 VDC (9-18 VAC, 50/60 Hz) 2 W
Led Test: All LEDs are tested on power-up
Enclosure: 1/8 DIN, high impact plastic, UL94V-0
Front Panel: NEMA 4X, panel gasket provided
Warranty: 3 years parts and labor
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