Midmark 417 Podiatry Foot Control Issues - Page 1
Forum section: electronics / repair

author: edy
author_rank: Super Contributor
timestamp: April 09, 2019, 05:44:44 am
content: Hi,A podiatrist friend of mine is having trouble with one of his chairs. He believes the foot pedal is to blame (not the chair itself). I assume he confirmed this by trying another pedal and cable which worked. Before I start messing around, I wanted to see if anyone here had any experience with this.The chair uses a telephone-type cable (RJ-11) in which there are 4 wires. As you can see by the photos below and service manual, the foot control has 11 buttons (AUTO RETURN, STOP, 1, 2, PROGRAM and then 6 more (2x3 up/down)). So it does not look to be a simple on/off bridge to each wire. There must be some signal generation in the pedal. I haven't opened it up yet but the service manual I linked below seems to show a board in the pedal.Does anyone have any idea how this thing might work and where I should start looking? Just to note... one of the buttons (labelled "1" seems to be stuck, it does not let me press down...I wonder if the foot control only allows one button to be pressed at a time or it can transmit to the chair when several buttons are pressed at the same time. Any ideas on where to start and how this works?https://www.boothmed.com/content/docs/manuals/midmarkparts/powertables/400series/417(-001-003).pdf

author: edy
author_rank: Super Contributor
timestamp: April 09, 2019, 06:13:43 am
content: I just opened up the pedal, there is a control board under the middle pedal. It has a few microchips on it, completely caked in dust. I'll try to just clean things up first and see if that helps. Perhaps there is a short or the dust is causing signal problems as it was completely coated in a fine grey powered dust. I'll also try to unstick that "1" button switch in case that is preventing the other buttons from creating a signal. Still, it seems like a complex design for a foot pedal. I'd be interested in just learning how it works and why they decided to do it this way. And how is the signal sent along the 4 lines? Are they using a 16-to-4 line encoder? But how would they get power to the foot control? If one line is to send power, you would have 3 return lines and a priority encoder would only be able to handle 8 buttons encoding to 3 lines (actually 7 buttons because by default when no buttons pressed that might correspond to 000, then each of the 7 buttons when pressed would give 001, 010, 011, 100, 101, 110, 111).

author: fzabkar
author_rank: Super Contributor
timestamp: April 09, 2019, 07:06:03 am
content: I would think that the foot pedal would have a slave microcontroller with an i2c interface, or some other 2-wire bus.

author: edy
author_rank: Super Contributor
timestamp: April 09, 2019, 12:24:32 pm
content: This image is before cleaning, I just wiped my finger on a couple chips to see what was there. I'll clean it up and post more and try to figure out what is there.

author: edy
author_rank: Super Contributor
timestamp: April 09, 2019, 12:46:47 pm
content: I've cleaned up the board a little, I need to get out the compressed air still... but you can see a couple of mc14021bcp (8 bit static shift registers) and a National 4538 Dual Precision Monostable Multivibrator. Those long red packages at the top of the board are resistor arrays? The data sheet for the shift register says they are used a lot for parallel to serial conversion which explains how they need only a 4 connector cable to the chair. Any more than that I don't know enough yet to understand... I need time to learn more, but seems like the buttons on the foot control essentially parallel input to the shift registers, which serialized the data and send it to the chair. The monostable vibrator would act as a clock cycle generator to run the chips at a certain step rate? Each time a button is pressed some pins would be high and others low on the shift register chips and it would read each and turn it into a serial output? It needs 2 because each one only takes in 8 parallel inputs and we need at least 11?