- Microsoft embedded Visual Basic
- Windows CE 2.11, 3.0, 4.0, 4.1, 4.2, 5.0
- Pocket PC 2000, 2002
- Windows Mobile 2003, 2003SE, 5.0
Although this article specifically addresses a problem that I had between a Symbol PDT8046 and the O’Neil Route Printer RP2000-8000, the issue and resolution should be applicable to any serial (COM) port based line printer.
If you have a raw ascii line printer, you can check to see if the printer works by echoing directly from the stdout into the COM port. Under DOS or the Windows command shell on the PC or Pocket DOS on Windows CE/Windows Mobile issue the following commands
This will send I am a fish and a line feed to COM1 at 9600bps with no parity. Note that Pocket DOS doesn’t support the “,r” parameter.
So now that you have ascertained that the printer actually works, when you use the Microsoft Communications Control (Microsoft Comm Control) for embedded Visual Basic (eVB) to send the same data, why is it that nothing happens what so ever.
Let us look at the VB
This is a logical flow of execution and basically what on the surface the DOS version is doing. Yet the message I am a fish will never arrive at the printer. In fact, the printer will not even acknowledge the presence of the message.
This code is in fact completely logical, and is right (for the O’Neil RP2000-8000 anyway). However after some frustration and experimentation I discovered that there are two important pieces of the puzzle missing, one of them strongly recommended and the other very much mandatory.
Issue 1: Sleeping Printer
The O’Neil printer will go into standby mode after a couple of minutes of idle time so as to conserve power. If the printer is in sleep mode and you send data to it, it probably will not be awake to receive information and send it to the print head fast enough; best case nothing happens and worst case less than half of your message gets printed.
The O’Neil hardware manual has a very short section on this. In order to wake the printer up (or ensure that it is awake) you need to send a string of ASCII NULL characters (0x00) at the printer. For ONeil line mode printers operating at 9600bps you need to send this string 6+ times. For 38,400bps printers you should send this 24+ times. In our VB world that means:
7 times is a charm, however the manual also states that you should wait 150ms before sending data to the printer. See the next section on implementing Sleep().
If you repeat your test now, your printer should at least wake up even if nothing else happens
Issue 2: Speed = Distance / Time
I could wax lyrical on this, but Ill keep it brief.
There are a couple of buffers at play in the form of the OS and the printer, the embedded operating system and the eVB runtime processor. All of which cause delays. It follows something similar to this
- User Sends Print Request
- eVB Runtime Parses the command and sends it to the native code of the machine
- Operating System queues it in RAM as it gets processed through and send on its merry way
- Operating System queues it in the COM port transmit (TX) buffer
- Printer receives the message and queues it in the print buffer (512 bytes I *think* on the RP2000-8000)
- Printer processes it forward to the print head and the line at the front of the queue is printed
eVB does not care about steps 3 – 6, neither is it aware of them. As far as eVB is concerned, it has finished with the msComm.Output() command as soon as it has received an OK from the operating system that the requests has been through the processor and the data is in a queue.
Referring back to our original code
What is happening? eVB thinks that everything has been delivered and shuts down the COM port before the data has left the sending device and is safely received into the printers hardware buffer.
What is the solution? Well, you can be as eloquent or in-eloquent as you like. You can do a byte by byte check of data into the VB transmit buffer, ensuring that this has gone before you progress through the output stream. Alternatively if you are feeling lazy you could use the threading model and implement a fixed length sleep action. If however you are feeling somewhere between the two extremes you can try and weight the sleep timer based upon message length. Something along the lines of
delay (ms) = Absolute(((Length(Message) / ((Printer Bit Rate * 0.50) / 8 )) * 1000))
A weighting of 0.66 is assumed so that the link speed and buffer writing can be running at only 1/2 of line speed. Division by 8 is used to convert from Bits Per Second to Bytes Per Second. Multiplication by 1000 converts the value to milliseconds.
or for the RP2000-8000 with message “I am a fish <cr><lf>” [13 characters]
delay (ms) = Abs(13 / ((9600 * 0.66) / 8) * 1000)
or 21 ms
Onto this you may want to add a standard value that reflects the time the message takes to leave the operating system onto the wire. 50 – 100ms should do.
In order to use the thread sleep function in eVB you have to import the export from the C++ corelib as follows
Note: This code is highly summarised it will not work directly as a copy/paste, you need to modularise it and create the msComm object.
With the sleep function in place and the correct tuning you should now find that the data gets to the printer before eVB closes the COM port.
Remember that you need to implement the function of Sleep(150) after sending the wake-up NULL string to the printer.