Cefiar

Not only 2 modems, but 2 modem posts! Here's the inside of the Microcom Deskporte FAST modems from the last post.

Inside of the plastic case is coated in a brown conductive paint. The modem chipset is a Rockwell 288DPi, using what appears to be a Zilog Z80 to manage everything. The main difference between them seems to be where the components have been sourced from.

#ModemMonday #Modems #RetroComms #RetroComputing

Sep 11, 2023 at 2:14amWeb
Show thread Kale Sep 11, 2023
@cefiar what's with the conductive paint? Earthing? Shielding?
Show threadCefiarSep 11, 2023
@DarkestKale Shielding. Basically a combination of protecting the device from outside noise, but also a lot of protecting the outside from RF interference created by the internal circuitry, particularly in the 500 kHz to 2 MHz range (ie: good ol' AM radio). Quite common in a lot of older kit with plastic casings. PCB's with multiple layers (eg: a ground plane built into the board) and specific EMI/RFI shielding cans (ie: small metal boxes mounted over the specific offending circuitry and grounded on the PCB) help reduce this, so a lot of later designs don't do this as much. Still crops up from time to time for certain applications tho, and weighs a lot less than a metal case.
Show thread Kale Sep 11, 2023
@cefiar figures, yeah. Was wondering if this was a plastic case thing.