Quick Start to connect multiple devices (ASI)

This section is for ASI devices see below for VT100 devices

Connection

The ASI electrical specification allows for two separate types of input, one from a standard PC com port and the other from a UART - or a BV101-1 or BV201.

The 5 pin connector has TWO inputs and one output.

The input on pin 1 is designed to be connected directly to the output of a Microcontroller UART or to the TX pin of the BV101-4 It expects +5V and 0V to define the signal where +5V represents the idle no signal state, the first start bit will take this line low.

The BV101-4 is a USB to serial convertor, it installs a COM port on the PC and allows the COM port to be used as if it were a microcontroller UART (Universal Asynchronous Receiver Transmitter). This is a good option for controlling the BV ASI devices from a PC as it allows more than one to be connected to the same bus and as a bonus it supplies power!

 

The input on pin 4 can tolerate + and - 15V which is the standard output for an RS232 port (COM port) on a PC. Although the voltages vary considerably and are usually between + and - 6V to 12V. The idle state is negative, say -12V and the first start bit will take this line high so not only are the voltages different but they are opposite to each other.

There is only ONE output and this is capable of sending signals suitable for RS232 (normal) or for a UART (inverted), this is done in software be using command 4. Nearly all RS232 ports will accept 0V instead of the negative voltage and +5V instead of the +12 or +15V and so this TX pin will not output negative voltages so it is compatible with RS232 and UART.

At reset all devices are configured to send out normal signals from pin 2 so connecting to the RX pin of a COM port will work first time. If the device is connected to a UART or BV101 / 201 then command 4 will be needed to invert the output and from then on all of the output will be compatible with the UART.

Connecting Multiple Devices

The Input

If using a PC COM port (RS232) then simply connect multiple devices together using pin 4, TX on the master goes to RX on the slave ASI devices.

If using a UART or a BV101/201 then use pin 1, TX on the master goes to RX on the slave ASI devices.

The Output

As previously mentioned there is only one output and so this must cater for both RS232 and UART connections. The invert / non-invert is taken care of by software but for multiple devices only the UART option is available. This is because the TX output from the device disconnects itself when not transmitting to allow the other devices to take control of the bus and this can only be done of the idle state is high (+V) which it is for the UART output.

There is a pull up resistor connected to the circuit on the TX line and this is semi-permanently connected to +V via a thin PCB track shorting out jumper 1. The value of the pull up resistor is approximately 4k7 and connecting 3 devices together reduces this value about 1k5. This will work in most circumstances depending on the quality of signal and wiring however if problems occur or more then 3 devices are connected then the track needs cutting on each device except one of the devices.

The Address

Before connecting multiple devices together the address for each must be changed so that they can be uniquely addressed. This is done on an individual bases, connecting singly each device in turn to change its address. The device will retain its address until a factory reset is performed or the address is changed in software again.

To change the address from 'a' to say 'd' the following commands are required:

aU

aAd

The first command unlocks the device and the second changes the address to 'd'. The unlock command is only needed for changing the address and possibly factory reset.

VT100 Devices

These devices differ from the ASI devices in that they are not addressable. The main advantage of this is that anything going to the serial connection can be immediately resolved on the device making it easier to use. The disadvantage is that when connecting multiple devices the display not being addresses needs to be disabled.

Serial Connector

The serial connector is a 2x5 pin at the back of the device. Pin 1 is marked with 1, odd pins are down one side and even pins down the other.

Pin	Function	Function	Pin
1	RX	nDD	2
3	TX		4
5	+5V		6
7		nCTS	8
9	Gnd	nRTS	10

This bVT100 serial connector will mate directly with the BV103. For correct operation the RTS  & CTS lines are required and would normally be connected to the host.

This is the generalised VT100 connector, the actual device may vary slightly so to be sure consult the datasheet for that device. The all share the nDD pin which is the main subject of this topic.

nDD

The DD here is for Device Disable and the n means that it has to be taken low to do it. If this pin is left unconnected then it is pulled to +V by an internal resistor and so by default the serial interface on the device is enabled, it it is taken low then the serial interface on the device is disabled. This does not effect the contents on the device, it just effectively disconnects the display from the serial interface.

To use multiple devices in a microcontroller system it is simply a question of connecting all of the TX/RX lines in bus fashion and then taking a deselect line to each of the devices.

Above would be a typical set up for a microcontroller system. Each display would need a separate select line as shown. At any one time all of the select lines except one would be low, the one that wasn't low would be the display serial interface that is active.

Multiple Displays on a PC

It would be possible using a BV101/2/3 to connect two displays to the same BV101/2/3 by using the DTR output from the PC if this were not being used by the VT100 device, most don't use this but check the datasheet.

BV101 - This is the later version of the device (October 2009 on) that has the control lines as part of the socket.

The above shows that the DTR line could be used to control more than one display but additional electronics would be required so that when one nDD is low the other is high, setting the DTR line high or low would select one of the other display

Another alternative would be to use the CBUS4 line form the FTDI chip which has been brought out to pin 2 on the BV101/2/3. This would require software control of this pin. For further details see http://www.ftdichip.com/ on how to drive this pin with software. This particular method incidentally has not been tried in practice although the theory is sound