Models

Monolog was produced in 2 variants; Mk1 and Mk2.   The colour of the enclosure identifies the model.  The Mk1 units are housed in a Blue plastic enclosure, whilst the Mk2 variants, came in a Grey enclosure.  These are also usually Grey plastic, but I have also seen a metal variant also.

Internally, the Mk1 & Mk2 units are identical and the only difference between them is the EPROM firmware.  The earlier Mk1 units are easily accessible with nothing more than a basic serial communications package such as HyperTerminal, PuTTy or SecureCRT and a dial-up modem.  Once you had logged onto a Mk1 Monolog,  it operated simply as a Human interface, with all commands transmitted/received, essentially in cleartext and consequently no additional software was required.  

One can only assume in an attempt to improve the security of the unit and prevent them being logged onto remotely by any un-authorised users, the Mk2 was released which applied a basic level of obfuscation (security through obscurity).  Mk2 commands are sent as;

[count] [command] [byte(s)] [checksum]

and results are returned as;

[count] [type] [byte(s)] [checksum]

command and type are byte values with valid commands being $00 to $1F.  These reference a command vector table which is copied to RAM at startup.  Return types have bit 7 set for errors.   The Dialog host software running on the PC would perform error checking and conversion of the necessary values to binary, before transmitting them.