PRT TTL Output Values

[pat_wardell]

Hi, I opreate and do engineering on a PRT for someone and I was thinking of controlling the On/Off of the spindle motor (3/4 hp single phase 3450 rpm, they just do foam) with one of the outputs. I was thinking of using an SSR (solid state relay) but they typically require 10ma to turn on.

I read that the outputs were TTL. Are the outputs standard TTL? I was wondering what the actual sink and source specs were for the outputs? Are the open collector with pull up resistors?

Thanks,
Pat

[Gary Campbell]

Pat...
I am not sure on the PRT, but on the PRS the outputs are sourced. Depending on the board version, you may also have other options.

IF Mike R. sees this he will give the best answer.

[ken_rychlik]

The manual of the prt had a schematic for a simple relay system with all the parts available at radio shack.

It uses a 12 volt relay driven by a 5 volt source. It is very simple and easy to make.

I built several and they are working fine on my machine.

Dig through your manual or you can download the old manuals on the shopbot web site also.

[ken_rychlik]

Here is a copy of the shopbot page with the relay on schematic.

Kenneth

[dana_swift]

Patrick- TTL stands for "Transistor-to-Transistor-Logic". Actually TTL is effectively extinct. What the spec is is "LS-TTL" which stands for "Low-power Shottky TTL". I expect that distinction didn't help you immediately. However that is a specific definition of the output levels and current capabilities. What is specifies is: The output will "pull down" as much as 8 milliamps at 0.35v. (logical zero) The output will "source" as much as -0.4 milliamps at 2.7 v. (logical one) Source (Digikey/Fairchild Semiconductor DM74LS00N-ND) The general characteristics of a logic family may be exceeded by any given device. What is important to notice is that the capability to "pull to logical zero" is much greater than the ability to "push to logical one". This asymmetry causes some odd design characteristics. It is preferred that a TTL circuit be in the "one" state most of the time, because it takes more power from radio interference to convert a logical one to a logical zero than the other way around. There are many ways of hooking digital signals to each other, here is a page listing many of them, just "FYI": http://en.wikipedia.org/wiki/Logic_family Solid state relays are available that accept LS-TTL logic levels directly. If you have not found one keep looking. Be sure your solid state relay is designed for an inductive load, some aren't. Hope that helps- D

[richards]

The problem is the TTL specification. As far as I can tell, on my PRT-Alpha, each output can source or sink 24mA. True TTL can sink 8-10mA, but it can only source about 10% of that value.

All of the SSRs that I've used (solid state relays) have worked with as little as 4mA. The switching speed is slower as the voltage drops, but all of them have been reliable at 4mA.

All that really happens inside a SSR, on the control side, is that you turn an internal LED on and off. The output side of the SSR detects the LED and uses that detection to turn on the outputs.

To test the circuit using less than $2 in parts, just hook up a standard RED LED wired in series with a 330 ohm 1/4 watt resistor. Connect one end of the resistor to +5VDC. Connect the other end of the resistor to the LED's anode. Connect the Cathode end of the LED to the controller's output. If the LED turns on and is fairly bright, then the circuit will drive a SSR. That test uses the controller output to sink current.

The second test is to connect one end of the resistor to the controller's output , the other end of the resistor to the anode on the LED and the cathode of the LED to the ground. If the LED turns on brightly, then the controller is able to source sufficient current to operate an SSR.

Note than in both tests, the Anode end of the LED is pointed towards the higher voltage and the Cathode end of the LED is pointed towards the lower voltage. The flat side of the LED is the Cathode. If the LED has no flat side, the shorter leg is the Cathode.

If the LED only turns on dimly, then you will need to amplify the current with a transistor. It would be easier to Google simple circuits than to explain the connections, but if you email me, I can draw up a simple schematic that would show how to use a transistor to amplify the current.

[pat_wardell]

Here is a copy of the shopbot page with the relay on schematic.

Kenneth

Kenneth.... Thanks.

Looking at the schematic I see they are using Mosfet which is a very low power input to power the relay. This tells me the outputs on the PRT are low current outputs. I'm glad to see they are using a diode accross the relay coils to control the inductive spikes.

Pat

[pat_wardell]

Patrick- TTL stands for "Transistor-to-Transistor-Logic". Actually TTL is effectively extinct. What the spec is is "LS-TTL" which stands for "Low-power Shottky TTL". I expect that distinction didn't help you immediately. However that is a specific definition of the output levels and current capabilities. What is specifies is: The output will "pull down" as much as 8 milliamps at 0.35v. (logical zero) The output will "source" as much as -0.4 milliamps at 2.7 v. (logical one) Source (Digikey/Fairchild Semiconductor DM74LS00N-ND) The general characteristics of a logic family may be exceeded by any given device. What is important to notice is that the capability to "pull to logical zero" is much greater than the ability to "push to logical one". This asymmetry causes some odd design characteristics. It is preferred that a TTL circuit be in the "one" state most of the time, because it takes more power from radio interference to convert a logical one to a logical zero than the other way around. There are many ways of hooking digital signals to each other, here is a page listing many of them, just "FYI": http://en.wikipedia.org/wiki/Logic_family Solid state relays are available that accept LS-TTL logic levels directly. If you have not found one keep looking. Be sure your solid state relay is designed for an inductive load, some aren't. Hope that helps- D

Dana thanks for al the work...

Yes TTL is out of date but I think the manual I read online for the PRT stated TTL. I was hoping it was open collector which would mean they had a resistor to the collector of the TTL ouput transistor allowing the designer to provide a higher output (source) current. Yes TTL can sink (pull to ground) more than they can source (pull high).

I think the "asymmetry" you were speaking of is considered hysteresis which is beneficial in a nosiy environment unlike CMOS which changes state typically at the same voltage (midpoint of 0V and the supply voltage).

I guess I should go to where the machine is and see if I can determine which chips they are using for the output drivers.

Thanks for the input on the SSR. My concern with the SSR is that they are opto-isolated meaning they use an LED to turn on the SSR. Typical mfg. specs are ~10ma to turn on the SSR and TTL is typically .4ma output. This is what prompted my question.

I have done some communication with major SSR manufacturers for another project. Here are their recommendations for a capacitor start single phase 3/4-1 HP 120V motor if anyone is interested. For an inductice load such as an electric motor they suggest a "Random" type SSR and not a "Zero Crossing" type. A 1 HP 120V motor runs at full load around 12-15amps but they suggested a 50 amp SSR due to the inrush current that may be 50-100amps when the motor starts.

Thanks again, Pat

[pat_wardell]

The problem is the TTL specification. As far as I can tell, on my PRT-Alpha, each output can source or sink 24mA. True TTL can sink 8-10mA, but it can only source about 10% of that value.

All of the SSRs that I've used (solid state relays) have worked with as little as 4mA. The switching speed is slower as the voltage drops, but all of them have been reliable at 4mA.

All that really happens inside a SSR, on the control side, is that you turn an internal LED on and off. The output side of the SSR detects the LED and uses that detection to turn on the outputs.

To test the circuit using less than $2 in parts, just hook up a standard RED LED wired in series with a 330 ohm 1/4 watt resistor. Connect one end of the resistor to +5VDC. Connect the other end of the resistor to the LED's anode. Connect the Cathode end of the LED to the controller's output. If the LED turns on and is fairly bright, then the circuit will drive a SSR. That test uses the controller output to sink current.

The second test is to connect one end of the resistor to the controller's output , the other end of the resistor to the anode on the LED and the cathode of the LED to the ground. If the LED turns on brightly, then the controller is able to source sufficient current to operate an SSR.

Note than in both tests, the Anode end of the LED is pointed towards the higher voltage and the Cathode end of the LED is pointed towards the lower voltage. The flat side of the LED is the Cathode. If the LED has no flat side, the shorter leg is the Cathode.

If the LED only turns on dimly, then you will need to amplify the current with a transistor. It would be easier to Google simple circuits than to explain the connections, but if you email me, I can draw up a simple schematic that would show how to use a transistor to amplify the current.

Mike thanks! I was at home asking this question and was hoping someone had the answer on the PRT specs off the top of their head. I have designed quite a few circuits in my time but was hoping an actual spec would allow me to shortcut digging into the machine or designing a circuit to fit my needs. I am working off the KISS methodology (Keep It Simple Stupid). I guess I will take my DMM to the site and do some current measurments. I don't want to over tax the TTL so I can maintain reliability.

My research indicates ~10ma to fully turn on the opto-isolation of an SSR which the TTL may not provide.

I will post my test results on what the outputs can source when I'm done.

Thanks..... Pat

[richards]

Pat,

You'll probably find that you don't need 10mA to operate an SSR. A few years ago when I visited Opto-22, a company that makes Solid State Relays and other opto-coupled devices, a designer told me that many people drive SSRs directly from the outputs of micro-controllers that could only sink 2mA. He cautioned me that turn-on time might take a few milliseconds, but that the SSR would work.

I tried some tests using 5VDC and a 2.2K resistor in series with the control side of an SSR. Everything worked fine.