modification of a basic rotameter-type flow device to provide an electrical signal for monitoring fluid flows.
liquid) flows can be monitored if the photocell is placed at the bottom of the tube and the light source is placed a centimeter or two from the bottom of the tube. Flows as low as 0.01 cc/min were easily monitored with gas rotameter tubes not designed for such low flows. In summary, this report describes a simple, inexpensive
RECEIVED for review September 21, 1970. Accepted October 29, 1970. This work resulted from a project supported by Quantachrome Corp., Greenvale, N. Y.11548.
Low Cost Parallel-to-Serial Converter for Digital Instrumentation Morteza Janghorbani, John A. Starkovich, and Harry Freund Department of Chemistry, Oregon State University, Corvallis, Ore. 97331 COMMONLY AVAILABLE LABORATORY digital instruments, such as digital voltmeters, typically store information in parallel binary-coded-decimal (BCD) form. Long term storage of such data on paper tape, magnetic tape, or direct processing by a computer frequently requires conversion of the data into a serial form. Typical commercial units may cost between $1000 and $2000, and are usually designed as part of more complex subsystems, with special input-output characteristics that may not be readily compatible with common digital instruments. A relatively low cost, yet versatile converter, was designed using readily available TTL logic IC’s. Specifically, Figure 1 shows a block diagram of the converter employed to interface a chemical instrument supplying parallel BCD data with a teletype requiring serial ASCII Code. The parallel-to-serial converter accepts BCD data from the output of the chemical instrument aia a DIGIT SELECTOR section. The parallel BCD data are then serialized, encoded, and are outputted cia the OUTPUT STAGE onto the storage device. Figure 2 presents a functional diagram of the parallel to serial converter. The heart of the converter consists of three 4-bit static shift registers (Fairchild, U6B930051X) which function both as serializer and ASCII Encoder. The BCD-outputs of the chemical instrument are hard wired to the appropriate inputs IN1 to IN8; IN1 corresponding to the most significant digit. IN9 is wired to generate the BCD representation of an ASCII blank (space). IN1 thru IN9 each contains a quad 2-input AND gate (Motorola, MC3001P) and four high speed diodes (1N662 Jan). The four outputs of each of the quad AND gates are hard wired by means of the diodes to the four inputs of the shift registers PI thru Pl, the most significant binary bit (23) being connected to P4. Po and P5 thru PI1of the registers (not shown in Figure 2) are hard wired to 1’s or 0’s to provide the remaining bits of ASCII Code. Figure 3a shows the ASCII bit pattern for decimal three. When the start button is pressed (either manually or automatically) the data available at IN1 are fed to the inputs of registers and are shifted together with Po and Ps through PI1, one bit at a time at the clock rate of 9.1 milliseconds per bit to the output. After the 12 bits have been shifted out, the counter-decoder (Fairchild, U6A998979X and Fairchild, U6B930159X) counts up by one, making the contents of IN2 available to the registers and this cycling continues until the contents of IN9 have been serialized and shifted out. In automatic mode, the P-S converter is reset and placed in standby condition awaiting the next start pulse. The start signal (print command) is a 9-volt square wave whose duration is not critical. No stop signal is required. In manual mode, the entire scanning operation repeats continuously, with two temporary contact switches controlling start and stop operations. When the output is connected to a teletype, each cycle results in the print-out of eight digits followed by a space. A
u .-
iio
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
493
unter -decoder 12-bit shift register OUTPUT
IN9
t2.0
BCD inputs Figure 2. Functional diagram of P-Sconverter
I‘
I‘
‘2
‘3
5’
4‘
6‘
M p 3 w ‘ 5
1I X S - 3 gray code start bit
6‘
7‘
8‘
‘ 9
1 ‘0
1 bits
51
4
I
