Using Com3

[victorf]

I wish to use com3 to communcate with a device. The device I want to talk to implements an RS232 interface using a MAX232A. I am a bit confuses as to whether I should use inverted or non-inverted logic to configure Com3. The communications will be 9600 baud, 8 data bits, No parity and 1 stop bit. I wish to transmit on pin PC6 and receive on PC5.

I must admit that I am confused by how to prepare Com3. What routines need be called to get this going?

Any enlightenment will be appreciated.

Vic

[dkinzer]

First, a little background information. The RS232 standard calls for a logic 1 (also called "mark") to be transmitted as a level between -3 and -15 volts and a logic 0 (also called "space") as a level between +3 and +15 volts. A level converter like the MAX232(A) performs both inversion and level conversion. A +5 volt input on the TTL side yields a negative voltage on the RS232 side and a zero volt input on the TTL side generates a positive voltage on the RS232 side.

Since the device that you're connecting will be generating and expecting RS232 voltage levels, you need to add circuitry between the ZX and the device to perform the level conversion. A MAX232 is probably the easiest way to do this. Appendix C of the ZBasic Reference manual shows several alternatives to performing the level conversion.

Once you have the level converter in place, you'll want to select non-inverting mode for Com3.

Some devices have serial interfaces that don't conform to the RS232 standard but still work in most cases. The Com1 channel on the ZX-24 is an example of this. A mark is transmitted as zero volts and a space is transmitted as +5 volts. If you wanted to connect two ZX-24s using Com1 on one and Com3 on the other, you'd need to select inverting mode.

It is also possible to using inverting mode and a few resistors and diodes to connect to a device with a conforming RS232 interface. This should only be done if the connection is short (a few feet or so). To implement this interface, you need clipping diodes and a limiting resistor on the input to the ZX and a limiting resistor on the output from the ZX. Again, this is not recommended (because doing it incorrectly can damage the ZX) but it should work.

[dkinzer]

I neglected to answer the second part of your question.

To set up Com3 you have to prepare two queues, define the Com3 configuration and then open the port. Here is a code fragment that shows how it might be done.

[pjc30943]

Just a clarification question:

The docs say ComChannels has no effect on coms are already open... but I'm not clear which ports this is referring to: all of them, even ones to be opened in the future, or just ones that are already open (is it a compile-level statement, or runtime-level, etc.)

If I go from 3 ports to 2, for example, by closing a port, it may be benificial to redefine ComChannels for higher maximum baudrates. For this, _all_ coms must first be closed, right?
Or can ComChannels statement affect only the next port that is opened, just not those already in place? This still fits with the documentation's description, I believe.

[dkinzer]

[jorsam]

Hi,

Great chat and very clear how to define, set and open the serial communication using rxPin and txPin as rx and tx, etc.

But I have a basic question, sorry I'm a beginner:
Which command is used to send and receive serial communication using this open port?

Example: how to code a simple program to send and ASCII character to txPin and how to read a string from the rxPin?

Any help will be very much appreciated.

[dkinzer]