Hector Martin

Where might I start looking if I wanted to write an applet for communicating on an RS-485 bus?

This depends on what you mean by RS-485. RS-485 on paper is just a physical layer electrical standard, which Glasgow cannot correctly implement alone as it only has single-ended main I/O (LVDS port notwithstanding, but it's not protected/capable of this either)

However, when *most* people say RS-485 they mean "RS-232 style UART over RS-485". Glasgow already has a UART applet. So assuming you mean *that*: 1) If you need "true" RS-485, you need an external transceiver, and then the UART applet needs a minor change to add a TX enable pin.

2) if you just need to talk to "some" RS-485 device or a few in a controlled situation, you could further extend the UART applet to fake a RS-485 bus with two single-ended I/Os, assuming your grounds are common and sane, and get away without using a transceiver.

This will work because RS-485 is usually implemented using 0V/5V single-ended signal levels, and Glasgow supports 5V logic (that's in fact the maximum supported I/O voltage).

I’m principally interested in interacting with / observing SIA OSDP which is implemented as a serial 2 wire RS485 multi drop bus.

Right, so the question is whether you're talking to one or two devices with very short cables in a controlled tabletop lab scenario (then option 2 will probably work), or whether you need to talk to an actual bus with proper termination over a longer distance (then you need 1).

Probably 1, for the scenarios I’d like to experiment with.