Any restrictions on IO pins of Mega 2560?

Projects General Guidance
1

I have a project in mine, which requires switching two SP6T relays. The relays latch, and so have an "open" and "close" pin, so there are 24 pins on the relay. These need to be driven by open-collector drivers. I want to control the relays using a GPIB (IEEE-488) bus

The GPIB connector has 24 pins, but only 16 are important. I'd also like to drive some LEDs and read some switches.

I have the Mega 2560

has 54 IO pins. Are there any restrictions about these pins, for what they can and can't be used for? Are some reserved?

If these are totally under my control, I can probably implement what I want using those pins with some buffers and open-collector drivers. If some are reserved, I will probably have to use a couple of 4-16 decoders to cut down the number needed to drive the relays. Since each relays only needs to be in one of 6 positions, I don't need all 2^24 (=16777216) the combinations of "on" and "off" that the 24 lines would permit.

For anyone interested in what the project it, I want to extend the number of test ports on an HP 8720 20 GHz vector network analyzer by using an HP 8711A K12 switching test set. The latter takes two ports in, and connects either port of the VNA to any of 6 different ports.

Dave

2

i use arduino mega 2560 r3 and Arduino Wi-Fi Shield (antenna connector) i have interface cable and antenna

Yes and no. If you use them solely as GPIOs and don't use any other functionality of the microcontroller, there are no restrictions other than the electrical specification. Usually you want to do some debugging, though, so pin 0 and 1 go away for the serial connection to the PC. If you use any other hardware that uses for example the SPI bus another 4 pins go away because many of the GPIOs can have other functions configured for them.

3

I want to control the relays using a GPIB (IEEE-488) bus

Wow stone age computing :astonished:
I last used this bus in 1983.

Are there any restrictions about these pins, for what they can and can't be used for?

They can all be used for general purpose input output ( GPIO ) however some pins have additional special purpose usage like I2C, SPI, analogue and serial I/O.

Are some reserved?

Not as such but pins 0 & 1 are used in serial communications for uploading code and printing debug so avoid those if there is an alternative.

4

Most pins have secondary features.
This diagram is quite good: http://forum.arduino.cc/index.php/topic,146511.0.html
The atmel datasheet for the chip will explain all the pins.

Also if you use direct port manipulation, some pins require blocking from interrupts as they take more than one instruction to modify.
I think ports H,J,K, and L require this.

There are a few libraries that will take care of this for you, the slower Arduino digitalWrite will do this for you too.

5

Grumpy_Mike:
Wow stone age computing :astonished:
I last used this bus in 1983.

The GPIB bus has been around over 40 years, but it is sill included on a lot of high-end instruments.

A look at the data sheet
http://cp.literature.agilent.com/litweb/pdf/N5247-90002.pdf

of Agilent N5247A - 10 MHz to 67 GHz vector network analyzer, which will cost upwards of $200,000

http://www.home.agilent.com/en/pd-1898852-pn-N5247A/pna-x-microwave-network-analyzer?nid=-536902643.958546&cc=US&lc=eng

will show the N5247A comes with GPIB !! It is still used a lot.

Are there any restrictions about these pins, for what they can and can't be used for?

They can all be used for general purpose input output ( GPIO ) however some pins have additional special purpose usage like I2C, SPI, analogue and serial I/O.

Are some reserved?

Not as such but pins 0 & 1 are used in serial communications for uploading code and printing debug so avoid those if there is an alternative.
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Thank you for that. Someone else posted some restrictions too, so I will have to check these out.