Line drivers


Thread Starter

Loren Schreiber

I understand the concept of line drivers: a device to increase the distance a signal may be transmitted. I presume it needs both a transmitter and a receiver. Question: is there a standard voltage/current for such devices? I
have Mitsubishi vector drive with a line driver encoder output running at 5.5 volts. Is this any kind of standard? Do I need some kind of receiver to interpret this voltage, or if this is in the acceptable range of the PLC receiving the signal may I simply input the raw signal?

Loren Schreiber
Systems Integrator, MCued

Johan Bengtsson

What type of signal do you want to send over an increased distance?

If you are talking about some kind of fieldbus they contains two (or possibly more) pair of recievers and transmitters. Each pair is connected to one end. The signal recieved by one
of the recievers are transmitted on the other, in effect amplifying the signals in both directions.

You mention a vector drive and an encoder, well they could probably be anything that particular vendor have decided, someone else on the list can perhaps fill me in here.

A PLC receiving a digital input most often wants 24VDC, oh sure there are input circuits built for other voltages too, but you will probably not connect the signal from an encoder directly to a "normal" PLC input even if that is possible.

If this doesn't help you at all I simply don't understand your question.

/Johan Bengtsson

P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]

There are many different types of line drivers, depending on the application. But the "normal" output of most encoders is TTL level (>2.0 volts = 1, <.8 volts = 0, usually a 74xx output driver). There are also open collector drivers with pullup resistors (capable of sinking more current), and open collector drivers without pullups (capable of sinking more current and perhaps higher voltages). There are also differential drivers, which usually use an RS-422 type driver chip. If each channel has two wires designated A & A', B & B' etc., it's a differential driver. If there's only one wire per channel, it's single-ended and may or may not have a high-current driver.

If you have a differential encoder, don't have a noise/distance problem, and you want to put this directly to the PLC input, you may be able to do so - just leave one of each pair of outputs floating (not tied to ground). It will probably work better if you use the driver to pull the PLC input low, so tie the common PLC input to +5 volts.

Many encoder manufacturers actually specify the output device used. You may see designations such as "74LS04" (TTL), "2N3906" (open collector transistor), or "26LS31", "75174" or "3487" (differential driver).

Whether or not you can get away with putting encoder inputs directly into a PLC has other aspects. For example, what is the maximum frequency you will be driving? What is the frequency response of the PLC? What is the cable length and capacitance on the encoder? Just because ou get a nice-looking square wave when you turn the encoder by hand doesn't mean that it won't turn into a triangle wave at 10 KHz!


Willy Smith
Costa Rica

Guy H. Looney

I'm sure there are more qualified people than me on this subject, but I'll tell you what I know. Standard encoders typically use two 5VDC TTL
signals that are 90 degrees out of phase. Differential implies compliments (A+ & A-) & provides for longer transmission due to the fact
that the A & B channels are referenced to their own compliment rather than both being referenced to the same signal (typically a digital ground / 0VDC). Back in the day when I was working for a distributor, I dealt a lot w/ a company called BEI. They had both line drivers & open collectors. 99.99% of the time I used line drivers. As stated previously there are many flavors of line drivers. BEI had line drivers
that would allow you to have a square wave output equivalent to the input voltage you used (up to 24VDC). The reason higher voltages were offered is the same as why differential signals are available.....noise immunity. As far as I know, no special receiver is necessary. Customers that use higher voltages must make sure that the device monitoring the signal can handle the higher voltages. If not, they would have to adjust the input voltage level to an acceptable range.

The 5.5VDC level you mention isn't really that unusual. Yaskawa has the ability to "boost" their 5 VDC encoder signal to 5.5 VDC to help deal w/ voltage drop. Typically, there are margins for everything: main power requirement is 230VAC +10% / -15% for example. TTL voltage is no exception. Your question could be interpreted in 2 ways:

1) In terms of voltage level, is the signal in the acceptable range of the PLC?
If the PLC has a TTL input card then the voltage level is okay. If it's 24VDC level inputs, then you'll need to use a resistor network to boost the level.

2) In terms of frequency, is the signal in the acceptable range of the PLC?
That depends on the resolution of the encoder & the speed the motor (& encoder) is turning. Many PLC's have a counter card on board that will handle such signals. If the encoder's signal is above the maximum input frequency of the PLC, you'll need to go w/ a high speed counter card which could be fairly expensive depending on the PLC you're using.
As a refresher course, the maximum input frequency of the card is in Hz, which is equivalent to counts/second. If your encoder has 4000 counts (make sure you check to see that the number is the post-quadrature number) per rev & your moving 600 rpm, the output frequency is 40KHz. If the PLC can handle this frequency, you're in business. If not, I can think of 3 choices (I'm sure there are probably more):
a) Slow down the motor
b) Buy a high speed counter
c) Check to see if the motor / drive manufacturer has a scalable output feature. Many of today's modern servo systems allow you to specify what resolution you want the system to output. If you have such a system, you could lower the resolution & thus lower the output frequency.

Hope this helps.


Guy H. Looney
Motion Control Engineer

A.C.E. Systems, LLC
170 Medearis Drive
Old Hickory, TN 37138
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